readme: add a "contributing" section, mentioning new irc

README.rst

@@ -145,3 +145,21 @@ Android
 -------
 
 See :code:`contrib/android/Readme.md`.
+
+
+Contributing
+============
+
+Any help testing the software, reporting or fixing bugs, reviewing pull requests
+and recent changes, writing tests, or helping with outstanding issues is very welcome.
+Implementing new features, or improving/refactoring the codebase, is of course
+also welcome, but to avoid wasted effort, especially for larger changes,
+we encourage discussing these on the issue tracker or IRC first.
+
+Besides `GitHub`_, most communication about Electrum development happens on IRC, in the
+:code:`#electrum` channel on Libera Chat. The easiest way to participate on IRC is
+with the web client, `web.libera.chat`_.
+
+
+.. _web.libera.chat: https://web.libera.chat/#electrum
+.. _GitHub: https://github.com/spesmilo/electrum