Merge pull request #5 from stefanv/how-to-release

Add contributor README

Author: stefanv

CONTRIBUTOR.md

@@ -0,0 +1,7 @@
+## To release
+
+1. Update the version in `pyproject.toml`; commit
+2. Tag the commit with the same version: `git tag -s 0.2`
+3. Push the commit and tag: `git push --tags origin main`
+4. Update the version in `pyproject.toml` to the next development
+   version; push