RELEASE.md

The release process for XcodeGen

Make sure CHANGELOG.md is up to date:
- All relevant entries have been added with the PR link and author
- The new version number is added at the top after Master
- The Commits link is at the bottom of the release
Update the version at the top of Makefile
Run make release
Push commit and tag to github
Create release from tag on GitHub using the version number and relevant changelog contents
Run make archive and upload xcodegen.zip to the github release
Run make brew which will open a PR on homebrew core