Add RELEASING.txt

Add minimal document that describes how to make next zodburi release.
As suggested by @jamadden zest.releaser, by default, works ok most of the time.

MANIFEST.in

@@ -3,6 +3,7 @@ prune docs/_build
 graft zodburi
 
 include README.rst
+include RELEASING.txt
 include CHANGES.rst
 include contributing.md
 include CONTRIBUTORS.txt

RELEASING.txt

+========================
+ Making zodburi release
+========================
+
+To make a next release, by default, command ``fullrelease`` from
+`zest.releaser`__ works fine. It will go through to update version number,
+changelog, commit and tag those changes, upload the package to PyPI, and to
+further switch the source tree for the next development cycle.
+
+If/when needed, one could manually make all the steps described above.
+See `Packaging Python Projects`__ and zodburi git history for details.
+
+__ https://pypi.org/project/zest.releaser/
+__ https://packaging.python.org/tutorials/packaging-projects/