Update README to use stestr instead of testr for unit tests

This commit updates README.rst to use stestr instead of testr for unit
tests. The patch[1] for changing to use stestr was already merged. It's
good to change the document to use stestr instead of testr for the unit
test section at least.

[1] Iff4abef50178bdc83b868eed4a906e22d790762b

Change-Id: Ib6485d8281aaa5991188ea5c1ba9a6a4c827e8a6
1 file changed