README: add ruby 1.8.6 requirement

For now, it seems that homebrew-core requires Ruby 1.8.6

Known limitations of Ruby 1.8.5:
* RUBY_PATCHLEVEL is not defined
* instance_variable_defined? is not defined
* pathname.rb:223 - 'Digest::SHA2' => 'Digest::SHA256' is necessary

closes Linuxbrew/legacy-linuxbrew#12

README.md

@@ -17,6 +17,10 @@ What Packages Are Available?
 3. Or run `brew server` to browse packages off of a local web server.
 4. Or visit [braumeister](http://braumeister.org) to browse packages online.
 
+Requirement
+-----------
+**Ruby** v1.8.6 or newer.
+
 More Documentation
 ------------------
 `brew help` or `man brew` or check the Homebrew [wiki](https://github.com/mxcl/homebrew/wiki).