Spelling mistake and add links.

doc/install/installation.md

@@ -113,7 +113,7 @@ Then select 'Internet Site' and press enter to confirm the hostname.
 
 # 2. Ruby
 
-The use of ruby version managers such as RVM, rbenv and chruby with GitLab in production frequently lead to hard to diagnose problems. Version managers are not supported and we stronly advise everyone to follow the instructions below to use a system ruby.
+The use of ruby version managers such as [RVM](http://rvm.io/), [rbenv](https://github.com/sstephenson/rbenv) or [chruby](https://github.com/postmodern/chruby) with GitLab in production frequently leads to hard to diagnose problems. Version managers are not supported and we stronly advise everyone to follow the instructions below to use a system ruby.
 
 Remove the old Ruby 1.8 if present