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!

Revise o anúncio sobre o processo de desenvolvimento e o CHANGELOG.md na raiz, juntamente com as orientações para as branches master e maint/v0.22. Confirme se o repositório ainda documenta essas convenções e se é esperado que os pull requests atualizem CHANGELOG.md; considera-se concluído quando o processo atual está claramente documentado e consistente.