Adding information about gluster-selinux into docs

[schaffung]

On going through the docs it seems that we have information about the selinux xlator but I couldn't find anything on gluster-selinux. Probably we should add the information in here as it would improve the documentation and people who are wondering as to why geo-rep or any other service is affected when selinux rules are enforced can refer to these rules.

[sankarshanmukhopadhyay]

This is a good point - if you want to contribute a starter set of content - please set up a PR

[schaffung]

Yes, working on the patch, will give out a PR soon :)