Document adding SELinux context for CIFS mounts

[sartsj]

Using the documentation provided on github and http://netshare.containx.io/docs/cifs I struggled getting a CIFS mount to work in containers. The problem I was running into was that inside of the container I was not permitted to do anything on the mounted volume.

In the end it all seems to boil down to using the correct SELinux context when mounting the CIFS share. Something which is not mentioned in the documentation.

Perhaps it is a good idea to document that adding a context is probably needed when SELinux is enabled on the host machine.