State in the doc that the only supported Linux distribution is Ubuntu

[Marc]

I don't see anywhere that the only supported distribution is Ubuntu, only here. The installation fails on RHEL7 and CentOS 7 as explained on the issue #119.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/172734853

The labels on this github issue will be updated when the story is started.

[Anthony Emengo]

This is a good call. Sorry for any confusion.