enhancement: add a doc section about how to develop rules' plugins

Add a section into the doc about how to develop rules' plugins.

docs/development.rst

@@ -16,3 +16,47 @@ 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 rule plugins must satisfy the followings.
+
+#. It must be a Python package installable using pip and distributed under
+   GPLv3+ same as yamllint.
+#. It must contains the entry point configuration in ``setup.cfg`` or something
+   similar packaging configuration files, to make it installed and working as a
+   yamllint plugin like below. (``<plugin_name>`` is that plugin name and
+   ``<plugin_src_dir>`` is a dir where the rule modules exist.)
+   ::
+
+     [options.entry_points]
+     yamllint.plugins.rules =
+          <plugin_name> = <plugin_src_dir>
+
+#. It must contain custom yamllint rule modules:
+
+   - Each rule module must define a couple of global variables, ID and TYPE. ID
+     must not conflicts with other rules' ID.
+   - Each rule module must define a function named 'check' to test input data
+     complies with the rule.
+   - Each rule module may have other global variables.
+
+     - CONF to define its configuration parameters and those types.
+     - DEFAULT to provide default values for each configuration parameters.
+
+#. It must define a global variable RULES_MAP to provide mappings of rule ID
+   and rule modules to yamllint like this.
+   ::
+
+     RULES_MAP = {
+         # rule ID: rule module
+         a_custom_rule.ID: a_custom_rule
+     }
+
+To develop yamllint rules, the default rules themselves in yamllint may become
+good references.