docs: add developer nots with release procedure

DEVELOPER.rst

@@ -0,0 +1,16 @@
+====================
+Notes for developers
+====================
+
+Creating a release
+==================
+
+- Ensure that the ``version`` in ``serial_asyncio/__init__.py`` is updated
+  and in the form of major.minor[.patchlevel]
+- Ensure all changes, including version, is committed then create a tag with
+  the exact same value as the version (e.g. "0.5")
+- Push to GitHub, merge into master if it is a branch. The GitHub Actions
+  are set-up to build the default branch.
+- Inspect the built wheel and tar.gz files for correctness, test.
+- Trigger the upload to PyPi by using GitHub web interface "Releases" and
+  create a new release from the tag.