Add docs and requirements for selinux systems

For systems running selinux, the selinux package must be installed in
the virtualenv, otherwise it fails to properly find the required system
packages.  See ansible/molecule#1724 for
background.

README.md

@@ -112,6 +112,12 @@ mysql> show databases;
 4 rows in set (0.00 sec)
 ```
 
+> Note: If your system has selinux installed, you might see "Aborting, target
+> uses selinux but python bindings (libselinux-python) aren't installed!".  If
+> this happens, also install the dependencies in
+> `test-requirements-selinux.txt`.  See
+> https://github.com/ansible/molecule/issues/1724 for details.
+
 ## Built With
 
 ![Ansible](https://img.shields.io/badge/ansible-2.4.3.0-green.svg)

test-requirements-selinux.txt

@@ -0,0 +1,3 @@
+# This is required for molecule to work in a virtualenv on systems running
+# selinux.  See https://github.com/ansible/molecule/issues/1724 for details.
+selinux==0.1.3