RELEASING: Rework based on @stevepiercy suggestion

@stevepiercy says (https://github.com/Pylons/zodburi/pull/30#discussion_r630521139):

    This should be a plain text file, no markup. The intended audience are
    maintainers. It should not get included in user documentation. Plain
    text makes it easy to maintain and open links in editors that support
    that feature.
    ...

-> Rework the text based on that suggestion.
-> Keep pointers on how to do it manually for the reference.

RELEASING.txt

-========================
- Making zodburi release
-========================
+We use zest.releaser for release automation.
 
-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.
+https://pypi.org/project/zest.releaser/
 
-If/when needed, one could manually make all the steps described above.
-See `Packaging Python Projects`__ and zodburi git history for details.
+To make a new release, first prepare by adding any omitted entries to
+CHANGES.rst. Then use the following command, accepting all default options:
 
-__ https://pypi.org/project/zest.releaser/
-__ https://packaging.python.org/tutorials/packaging-projects/
+    fullrelease
+
+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.
+
+See zest.releaser documentation for details:
+
+https://zestreleaser.readthedocs.io/en/latest/overview.html#available-commands
+
+
+If/when needed, one could also manually make all release steps described above.
+See "Packaging Python Projects"
+
+https://packaging.python.org/tutorials/packaging-projects/
+
+and zodburi git history for details.