Add a simple example of a custom plugin to the documentation

Provide a simple but complete example of a custom step plugin not doing
anything more than defining a property.

docs/devel.rst

@@ -7,8 +7,58 @@ Extending and developing Bakefile
 Writing Bakefile plugins
 ------------------------
 
-.. TODO
+As mentioned in :ref:`loading_plugins`, it is possible to use plugins written
+in Python to extend Bakefile functionality. The most common types of plugins
+are those defining custom tool sets, i.e. new output formats, or custom build
+steps, allowing to perform more or less arbitrary actions.
 
+But the simplest possible custom step plugin may actually not do anything at
+all, but just define some properties that can be used in bakefiles. For
+example, here is a complete example of a plugin:
+
+.. code-block:: py
+
+    from bkl.api import CustomStep, Property
+    from bkl.vartypes import StringType
+
+    from datetime import date
+
+    class MyVersion(CustomStep):
+        """
+        Simple bakefile plugin defining MY_VERSION property.
+
+        The value of the version is just the current date.
+        """
+        name = "my_version"
+
+        properties_project = [
+            Property("MY_VERSION",
+                     type=StringType(),
+                     default=data.today().isoformat(),
+                     inheritable=False,
+                     readonly=True),
+        ]
+
+This plugin can then be used in the following way:
+
+.. code-block:: bkl
+
+    plugin bkl.plugins.my_version.py;
+
+    program my_program {
+        basename = my_program-$(MY_VERSION);
+        ...
+    }
+
+Of course, a more realistic example would use something other than just the
+date of the last Bakefile execution as version. As plugin is just a normal
+Python script, there are a lot of possibilities, for example it could extract
+the version from the VCS used by the program or read it from some file inside
+the source tree.
+
+.. TODO provide an example of implementing generate() in a custom step
+
+.. TODO explain what other plugin types can be used for
 
 Reference
 ---------

docs/language.rst

@@ -687,6 +687,8 @@ specified one.
 
 
 
+.. _loading_plugins:
+
 Loading plugins
 ---------------