bitbake: README: Improve explanation about running the testsuite

Explain how to run the testsuite clear enough so no external
documentation is required.

(Bitbake rev: 51cf7cf12d23b757aa49f428a256fb3d3c865f6d)

Signed-off-by: Frank de Brabander <debrabander@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/README

@@ -34,10 +34,21 @@ Source code:
 
     https://git.openembedded.org/bitbake/
 
-Testing:
+Testing
+-------
 
 Bitbake has a testsuite located in lib/bb/tests/ whichs aim to try and prevent regressions.
 You can run this with "bitbake-selftest". In particular the fetcher is well covered since
 it has so many corner cases. The datastore has many tests too. Testing with the testsuite is
 recommended before submitting patches, particularly to the fetcher and datastore. We also
 appreciate new test cases and may require them for more obscure issues.
+
+To run the tests "zstd" and "git" must be installed. Git must be correctly configured, in
+particular the user.email and user.name values must be set.
+
+The assumption is made that this testsuite is run from an initialized OpenEmbedded build
+environment (i.e. `source oe-init-build-env` is used). If this is not the case, run the
+testsuite as follows:
+
+    export PATH=$(pwd)/bin:$PATH
+    bin/bitbake-selftest