The release process for XcodeGen

1. Make sure CHANGELOG.md is up to date:
   • All merged PRs since the last release have been added with the PR link and author (check git log <last-tag>..HEAD)
   • The new version number is added at the top after Master
2. Update the version at the top of Makefile
3. Run make release
4. Run make archive
5. Push commit and tag to github
6. Create release from tag on GitHub using the version number and relevant changelog contents, attaching xcodegen.zip and xcodegen.artifactbundle.zip