Add documentation about white box/black box testing to HACKING

This patch set adds documentation about white box vs block box
testing and their relationship in Patrole. This is so that
devs/test writers understand that Patrole is a bit different than
Tempest and requires digging a bit deeper in the internals of
the API implementation in order to properly test RBAC.

Also removes a misleading link in the README.rst section. The
discussion on member vs. _member_ role is very outdated and
so a link is provided to the RBAC overview section instead which
is concerned with documenting such information.

Change-Id: I0a014c2e917caeb058dd5b5294dd0af2e5e49132
diff --git a/README.rst b/README.rst
index 2028536..b07b109 100644
--- a/README.rst
+++ b/README.rst
@@ -201,8 +201,10 @@
   **admin** and **member** roles. However, other services may use entirely
   different roles.
 
-For more information about the member role and its nomenclature,
-please see: `<https://ask.openstack.org/en/question/4759/member-vs-_member_/>`__.
+For more information about RBAC, reference the `rbac-overview`_
+documentation page.
+
+.. _rbac-overview: https://docs.openstack.org/patrole/latest/rbac-overview.html
 
 Unit Tests
 ----------