FIX for issue 15, documented issues during MarkUs installation on Linux

Added documentations (#15) for using rvm to install a previous version of Ruby, fixing openssl error, and `cannot load such file` error for `svn/repos` and `svn/ext/core`.

It'd be great if @mike-stewart or @yc2lee could take a look and see if there is anything that is incorrect, inaccurate or missing. Thanks!