Add section in FAQ for Ubuntu14.04

With the default ubuntu version (14.04) of jekyll building the site does not work. Added problem description and workaround in README.md.

Note: I see below that changes are marked for lines 45, 77. I can not figure out why that is, this PR is only for adding the section from line 316.
"This installs jekyll in `/usr/local/bin`" -> "This installs Jekyll in `/usr/local/bin`".

Maybe add a link or explain how can I set my `PATH` environment variable correct?