docs: Add Patrole RBAC field guide

This documentation update adds a README.rst to
patrole_tempest_plugin/tests/api which explains the RBAC field
guide. It is modeled after Tempest's API field guide [0][1]. The
README.rst is then referenced in a new field_guide section
under doc/source/field_guide.

[0] https://github.com/openstack/tempest/tree/28b252f7f61a2dfc585089265cdb3118defbcfff/tempest/api
[1] https://docs.openstack.org/tempest/latest/field_guide/api.html#api-field-guide

Change-Id: I877ce4a1f681bd483c7f71b02fd7bb2b4d3b3e2a
4 files changed