docs: Separate README.md and docs index, elaborate more

Have the index page be rationale/comparison for now.

Closes: #370
Approved by: jlebon

README.md

@@ -1,43 +1,26 @@
 # rpm-ostree Overview
 
-New! See the docs online at [Read The Docs (rpm-ostree)](https://rpm-ostree.readthedocs.org/en/latest/ )
-
------
-
 rpm-ostree is a hybrid image/package system.  It uses
-[OSTree](https://ostree.readthedocs.io/en/latest/) as an image format,
-and uses RPM as a component model.
+[OSTree](https://ostree.readthedocs.io/en/latest/) as a base image
+format, and supports RPM on both the client and server side using
+[libhif](https://github.com/rpm-software-management/libhif).
 
-The project aims to bring together a hybrid of image-like upgrade
-features (reliable replication, atomicity), with package-like
-flexibility (introspecting trees to find package sets, package
-layering, partial live updates).
+For more information, see the online manual: [Read The Docs (rpm-ostree)](https://rpm-ostree.readthedocs.org/en/latest/)
 
 **Features:**
 
  - Atomic upgrades and rollback for host system updates
  - A server side tool to consume RPMs and commit them to an OSTree repository
- - A system daemon to consume ostree commits as updates
+ - A system daemon to consume OSTree commits as updates
 
 Projects using rpm-ostree
--------------------------
+--------------------------
 
 [Project Atomic](http://www.projectatomic.io/) uses rpm-ostree to
 provide a minimal host for Docker formatted Linux containers.
 Replicating a base immutable OS, then using Docker for applications.
 
-Using rpm-ostree to build OS images/trees
------------------------------------------
-
-See [Compose Server](docs/manual/compose-server.md).
-
-Hacking
--------
-
-See [Hacking](HACKING.md).
-
-Contributing
-------------
-
-See [Contributing](CONTRIBUTING.md).
+Manual
+------
 
+For more information, see the online manual: [Read The Docs (rpm-ostree)](https://rpm-ostree.readthedocs.org/en/latest/)

docs/index.md

@@ -1 +0,0 @@
-../README.md

docs/index.md

@@ -0,0 +1,91 @@
+# What is rpm-ostree?
+
+rpm-ostree is a hybrid image/package system.  It uses
+[OSTree](https://ostree.readthedocs.io/en/latest/) as a base image
+format, and accepts RPM on both the client and server side, sharing
+code with the [dnf](https://en.wikipedia.org/wiki/DNF_(software)) project;
+specifically [libhif](https://github.com/rpm-software-management/libhif).
+
+# Getting started
+
+If you want to try the system as a user, we recommend
+[Project Atomic](http://www.projectatomic.io/). If you are
+interested in assembling your own systems, see
+[compose server](manual/compose-server.md).
+
+# Why would I want to use it?
+
+One major feature rpm-ostree has over traditional package management
+is atomic upgrade/rollback.  It supports a model where an OS vendor
+(such as [CentOS](https://www.centos.org/) or
+[Fedora](https://getfedora.org/) can provide pre-assembled "base OS
+images", and client systems can replicate those, and possibly layer on
+additional packages.
+
+rpm-ostree is a core part of the [Project Atomic](http://www.projectatomic.io/)
+effort, which uses rpm-ostree to provide a minimal host for
+Docker formatted Linux containers.
+
+We expect most users will be interested in rpm-ostree on the client
+side, using it to replicate a base system, and possibly layer on
+additional packages, and use containers for applications.
+
+# Why not implement these changes in an existing package manager?
+
+The [OSTree related projects](https://ostree.readthedocs.io/en/latest/manual/related-projects/)
+section covers this to a degree.  As soon as one starts taking
+"snapshots" or keeping track of multiple roots, it uncovers many
+issues.  For example, which content specifically is rolled forward or
+backwards?  If the package manager isn't deeply aware of a snapshot
+tool, it's easy to lose coherency.
+
+### Filesystem layout
+
+A concrete example is that rpm-ostree moves the RPM database
+to `/usr/share/rpm`, since we want one per root `/usr`.  In contrast,
+the [snapper](http://snapper.io/) tool goes to some effort to
+include `/var/lib/rpm` in snapshots, but
+avoid rolling forward/back log files in `/var/log`.
+
+OSTree requires clear rules around the semantics
+of directories like `/usr` and `/var` across upgrades, and
+while this requires changing some software, we believe the
+result is significantly more reliable than having two separate
+systems like yum and snapper glued together, or apt-get and BTRFS,
+etc.
+
+### User experience 
+
+Furthermore, beyond just the mechanics of things like the filesystem
+layout, the implemented upgrade model affects the entire user
+experience.
+
+For example, the base system OSTree commits that one replicates from a
+remote server can be assigned version numbers.  They are
+released as coherent wholes, tested together.  If one is simply
+performing snapshots on the client side, every client machine
+can have different versions of components.
+
+Related to this is that rpm-ostree clearly distinguishes which
+packages you have layered, and it's easy to remove them, going back to
+a pristine, known state.  Many package managers just implement a "bag
+of packages" model with no clear bases or layering.  As the OS evolves
+over time, "package drift" occurs where you might have old, unused
+packages lying around.
+
+# But still evolutionary
+
+On the other hand, rpm-ostree in other ways is very evolutionary.
+There have been many, many different package managers invented -
+why not adopt or build on one of those?
+
+The answer here is that it takes a long time for tooling to be built
+on top of a package format - things like mirroring servers.  Another
+example is source format representations - there are many, many
+tools that know how to build source RPMs.
+
+From the perspective of distribution which has all of that ecosystem
+built up, rpm-ostree does introduce a new binary format (ostree), but
+otherwise includes an RPM database, and also operates on packages.  It
+is not a new source format either.
+