docs: add release documentation

Author: viktomas

docs/release.md

@@ -0,0 +1,11 @@
+# Releasing `godu`
+
+`godu` is released using a GitHub action and `goreleaser`.
+
+1. tag a new version `git tag v1.5.1`
+1. push the tag `git push origin v1.5.1`
+
+If everything goes well, that's all you have to do.
+
+## Generate homebrew taps token
+Once a year, you need to [generate a new token](https://github.com/settings/tokens?type=beta) for the [homebrew-taps](https://github.com/viktomas/homebrew-taps) repository. So that the `goreleaser` job can write into it.