docs: document release process (#27)

Document release process in a separate file.

RELEASE.md

@@ -0,0 +1,26 @@
+Release process
+===============
+
+Create a pull request with a version bump to indicate the intention to do a release. This should give users the ability to give feedback, e.g. if a breaking change was missed and the version bump is incorrect.
+
+Once that PR is merged do the actual release with [`cargo-release`](https://github.com/crate-ci/cargo-release).
+
+This requires the following permissions
+
+- on github.com/vmx/temp-env
+  - creating tags
+  - pushing to `main`
+- on crates.io
+  - publish access to all published crates
+
+Dry run
+
+```console
+$ cargo release -vvv
+```
+
+Actual publishing
+
+```console
+$ cargo release --execute
+```