selftests: forwarding: README: Document customization

That any sort of customization is possible at all, let alone how it should
be done, is currently not at all clear. Document the whats and hows in
README.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Link: https://lore.kernel.org/r/e819623af6aaeea49e9dc36cecd95694fad73bb8.1711464583.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/README

@@ -56,3 +56,36 @@ o Checks shall be added to lib.sh for any external dependencies.
 o Code shall be checked using ShellCheck [1] prior to submission.
 
 1. https://www.shellcheck.net/
+
+Customization
+=============
+
+The forwarding selftests framework uses a number of variables that
+influence its behavior and tools it invokes, and how it invokes them, in
+various ways. A number of these variables can be overridden. The way these
+overridable variables are specified is typically one of the following two
+syntaxes:
+
+	: "${VARIABLE:=default_value}"
+	VARIABLE=${VARIABLE:=default_value}
+
+Any of these variables can be overridden. Notably net/forwarding/lib.sh and
+net/lib.sh contain a number of overridable variables.
+
+One way of overriding these variables is through the environment:
+
+	PAUSE_ON_FAIL=yes ./some_test.sh
+
+The variable NETIFS is special. Since it is an array variable, there is no
+way to pass it through the environment. Its value can instead be given as
+consecutive arguments to the selftest:
+
+	./some_test.sh swp{1..8}
+
+A way to customize variables in a persistent fashion is to create a file
+named forwarding.config in this directory. lib.sh sources the file if
+present, so it can contain any shell code. Typically it will contain
+assignments of variables whose value should be overridden.
+
+forwarding.config.sample is available in the directory as an example of
+how forwarding.config might look.