enhancement: add lint rules plugin support

Add plugin support using setuptools (pkg_resources) plugin mechanism to
yamllint to allow users to add their own custom lint rule plugins.

Also add some plugin support test cases, an example plugin as a
reference, and doc section about how to develop rules' plugins.

Signed-off-by: Satoru SATOH <satoru.satoh@gmail.com>
Co-authored-by: Adrien Vergé

docs/development.rst

@@ -16,3 +16,15 @@ Basic example of running the linter from Python:
 
 .. automodule:: yamllint.linter
    :members:
+
+Develop rule plugins
+---------------------
+
+yamllint provides a plugin mechanism using setuptools (pkg_resources) to allow
+adding custom rules. So, you can extend yamllint and add rules with your own
+custom yamllint rule plugins if you developed them.
+
+yamllint plugins are Python packages installable using pip and distributed
+under GPLv3+. To develop yamllint rules, it is recommended to copy the example
+from ``tests/yamllint_plugin_example``, and follow its README file.  Also, the
+core rules themselves in ``yamllint/rules`` are good references.