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.
"Source:http://michaelchelen.net/81fa/install-jekyll-2-ubuntu-14-04/" -> "Source: http://michaelchelen.net/81fa/install-jekyll-2-ubuntu-14-04/"