New Development Process

With the release of v0.22, we want to streamline the development processes a bit and make them more like the current libgit2 "development rooles". This will help those of us who work on both just a little bit saner, and hopefully also be more in line with what other projects do. This means:

1. The master branch is where development happens now. It's now the default branch and where new pull requests should be targeted.
2. Any bugfix releases we'll branch a maint/v0.22 to handle it, based on the appropriate tagged release.
3. Please make sure that your pull requests update the [CHANGELOG.md](https://github.com/libgit2/libgit2sharp/blob/master/CHANGELOG.md) at the root of the repository.

We'll deal with manually merging any existing PRs that we want to take. Thanks!

開発プロセスに関する告知とルートの CHANGELOG.md、および master ブランチと maint/v0.22 ブランチのガイダンスを確認してください。リポジトリがこれらの規約を引き続き文書化しているか、また pull request で CHANGELOG.md を更新することが求められているかを確認してください。現在のプロセスが明確に記録され、一貫していれば完了です。