DOC: add announcement mailing list information (#95)

Add information about the mailing list for announcements of new GLASS
releases.

Fixes: #66

README.md

@@ -44,8 +44,20 @@ e.g. a design decision or API change, you can use our [Discussions] page.
 
 We also have a public [Slack workspace] for discussions about the project.
 
+To keep up with new GLASS releases, you can receive GitHub release
+notifications from this repository.  Alternatively, you can subscribe to our
+announcement mailing list, which only receives 1 email/release.  To subscribe,
+use the [mailing list page], or send an email to `[email protected]` with
+any subject and the following message body:
+
+    subscribe glass <Your name>
+
+where `<Your name>` is your full name, or `ANONYMOUS` if you prefer. You will
+be sent a confirmation email in return.
+
 
 [documentation]: https://glass.readthedocs.io/
 [examples]: https://glass.readthedocs.io/projects/examples/
 [Discussions]: https://github.com/orgs/glass-dev/discussions
 [Slack workspace]: https://glass-dev.github.io/slack
+[mailing list page]: https://jiscmail.ac.uk/lists/GLASS.html

docs/manual/installation.rst

@@ -61,3 +61,19 @@ There currently are a fair amount of breaking changes between *GLASS* releases.
 For production use, it is therefore **strongly** recommended that you install
 *GLASS* in a clean environment (venv, conda, etc.) and **pin the GLASS version
 for the duration of your project**.
+
+
+Announcements
+-------------
+
+To keep up with new GLASS releases, you can subscribe to our announcement
+mailing list, which only receives 1 email per release.  To subscribe, use the
+`mailing list page`__, or send an email to ``[email protected]`` with any
+subject and the following message body::
+
+    subscribe glass <Your name>
+
+where ``<Your name>`` is your full name, or ``ANONYMOUS`` if you prefer. You
+will be sent a confirmation email in return.
+
+__ https://jiscmail.ac.uk/lists/GLASS.html