README-group.md

This patch adds suport for the IPA CLI option `posix` when modifying
an existing group. Also, enhances verification of `external` and
`posix/non-posix` groups to avoid unneded API failures (e.g. when
no change to the posix/external status is needed).

A new test was added:

    tests/group/test_group_external_nonposix.yml