Update README.textile

Added instructions for installing zsh (due to the problems with first time zsh users)
Update README.textile
Update rvm.plugin.zsh

Ruby 2.1.1 -> Ruby 2.1.2.
Merge pull request #2994 from admpsktt/patch-1

Update rvm.plugin.zsh
Merge pull request #2993 from mittalabhas1/master

Update README.textile
Add CONTRIBUTING.md file general structure
Categories of issues in oh-my-zsh (work-in-progress)