Update documentation after repository split.

.github/CONTRIBUTING.md

.github/ISSUE_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

Library/Homebrew/README.md

Library/Homebrew/cmd/create.rb

Library/Homebrew/cmd/help.rb

Library/Homebrew/cmd/tap-readme.rb

Library/Homebrew/formula.rb

Library/Homebrew/manpages/brew.1.md

Library/Homebrew/os/mac/xcode.rb

Library/Homebrew/test/test_cmd_info.rb

README.md

share/doc/homebrew/Brew-Test-Bot-For-Core-Contributors.md

share/doc/homebrew/Brew-Test-Bot.md

share/doc/homebrew/Common-Issues.md

share/doc/homebrew/External-Commands.md

share/doc/homebrew/Formula-Cookbook.md

...ow-To-Open-a-Homebrew-Pull-Request-(and-get-it-merged).md

share/doc/homebrew/Installation.md

share/doc/homebrew/New-Maintainer-Checklist.md