CONTRIBUTING: add a DCO

The new CONTRIBUTING file has a Developer Certificate of Origin with
instructions for contributors to ensure 'Signed-off-by:' line in the
commit log (git commit -s). Corresponding changes also done in Gerrit
to reject patches which do not have Signed-off-by: line.

Change-Id: Ia3e1e8d3cfd4b32e4cfd4d2df91c6dbd57e2f60e
BUG: 3234
Signed-off-by: Anand Avati <avati@gluster.com>
Reviewed-on: http://review.gluster.com/767
Tested-by: Gluster Build System <jenkins@build.gluster.com>

CONTRIBUTING

@@ -0,0 +1,25 @@
+        Developer's Certificate of Origin 1.1
+
+        By making a contribution to this project, I certify that:
+
+        (a) The contribution was created in whole or in part by me and I
+            have the right to submit it under the open source license
+            indicated in the file; or
+
+        (b) The contribution is based upon previous work that, to the best
+            of my knowledge, is covered under an appropriate open source
+            license and I have the right under that license to submit that
+            work with modifications, whether created in whole or in part
+            by me, under the same open source license (unless I am
+            permitted to submit under a different license), as indicated
+            in the file; or
+
+        (c) The contribution was provided directly to me by some other
+            person who certified (a), (b) or (c) and I have not modified
+            it.
+
+        (d) I understand and agree that this project and the contribution
+            are public and that a record of the contribution (including all
+            personal information I submit with it, including my sign-off) is
+            maintained indefinitely and may be redistributed consistent with
+            this project or the open source license(s) involved.