CONTRIBUTING.md: Add Contributing to Linuxbrew
CONTRIBUTING.md: Add Contributing to Linuxbrew

Describe when pull requests should go to Linuxbrew and when to Homebrew.

rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core