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.
I prefer "`can't convert nil into String` error. What is wrong?" because this problem can also happen in Debian and others Linux distributions based on it and using the same package.