Improve plugins.rst

docs/plugins.rst

@@ -2,4 +2,12 @@
 Plugins
 =======
 
-You can find an example in ``examples/plugins/`` directory.
+Let's Encrypt client supports dynamic discovery of plugins through the
+`setuptools entry points`_. This way you can, for example, create a
+custom implementation of
+`~letsencrypt.client.interfaces.IAuthenticator` without having to
+merge it with the core upstream source code. Example is provided in
+``examples/plugins/`` directory.
+
+.. _`setuptools entry points`:
+  https://pythonhosted.org/setuptools/setuptools.html#dynamic-discovery-of-services-and-plugins