install packages documentation

https://bitmask.net/en/install/linux talks about Ubuntu 15.04 and 15.10, the first and supported until 2016 Feb 04 and the latter until 2016 July 28th. Can we remove both? 16.10 will be EOL by July 2017, so not worth adding, but if we have packages for 17.04, we should add them.

Unsure about the milestone.