📚 Add compatibility policy to README.md

I saw a chart similar to this in ruby/openssl's README, and it seems
like this is a relatively simple way to communicate our versioning.

I'll propose documentation for a slightly more detailed policy in a
future PR.

Author: nevans

README.md

@@ -60,6 +60,45 @@ end
 imap.expunge
 ```
 
+## Compatibility and Maintenance Policy
+
+`net-imap` is bundled with Ruby releases.  As a [bundled gem], it can be
+uninstalled from a Ruby installation and must be declared in `Gemfile` when used
+with `bundler`.  Each Ruby `major.minor` release series will be pinned to a
+specific `net-imap` `major.minor` release series, and will only bundle releases
+from that series.  Each `net-imap` release series will receive security updates
+as long as they are bundled with [supported Ruby branches].  The latest stable
+Ruby release series may also receive backported bugfixes and features, at the
+maintainers' discretion.
+
+|net-imap |ruby           |maintenance level                     |
+|---------|---------------|--------------------------------------|
+|current  |main (dev)     |new features (and deprecations)       |
+|current-1|current stable |discretionary backported bugfixes, etc|
+|current-2|previous stable|security maintenance                  |
+|current-3|oldest stable  |security maintenance                  |
+|_older_  |_older_        |end of life                           |
+
+### Release series
+
+|     |Bundled with|Compatible with|First bundled release        |Supported?    |
+|-----|------------|---------------|-----------------------------|--------------|
+|0.5.x| _not yet_  | Ruby 3.1+     |_future Ruby 3.4 release_    |new features  |
+|0.4.x| Ruby 3.3   | Ruby 2.7.3+   |0.4.9, Ruby 3.3.0, 2023-12-25|some backports|
+|0.3.x| Ruby 3.2   | Ruby 2.6+     |0.3.4, Ruby 3.2.0, 2022-12-25|security only |
+|0.2.x| Ruby 3.1   | Ruby 2.5+/2.6+|0.2.2, Ruby 3.1.0, 2021-12-25|security only |
+|0.1.x| Ruby 3.0*  | Ruby 2.5+     |0.1.1, Ruby 3.0.0, 2020-12-25|end of life   |
+|_pre_| in stdlib  |               |       Ruby 1.6.2, 2000-12-25|end of life   |
+
+Version 0.1.x was packaged as a [default gem] for Ruby 3.0, and can not be
+uninstalled from a Ruby 3.0 installation.  Before being extracted into a gem,
+`net/imap` was only packaged as a part of Ruby's releases.  The first stable
+release to include `net/imap` was ruby 1.6.2, on 2000-12-25.
+
+[bundled gem]: https://docs.ruby-lang.org/en/master/standard_library_rdoc.html#label-Bundled+gems
+[default gem]: https://docs.ruby-lang.org/en/master/standard_library_rdoc.html
+[supported Ruby branches]: https://www.ruby-lang.org/en/downloads/branches/
+
 ## Development
 
 After checking out the repo, run `bin/setup` to install dependencies. Then, run `bundle exec rake test` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.