Add API docs.

docs/api.rst

+API
+===
+
+`diceware` code is geared towards commandline usage. You can, however,
+use it from Python. The API docs are here to assist you with that.
+
+For using `diceware` in your own, `setuptools`-based Python project,
+you can add it as an install requirement in ``setup.py`` of your
+project::
+
+  from setuptools import setup
+  # ...
+  setup(
+      name="myproject",
+      # ...
+      install_requires=[
+          #  packages we depend on...
+          'setuptools',
+          'diceware',
+          # ...
+      ],
+      # ...
+  )
+
+Of course there are other ways to make `diceware` available.
+
+
+`diceware` main module
+----------------------
+
+.. automodule:: diceware
+   :members:
+
+`diceware.wordlist`
+-------------------
+
+.. automodule:: diceware.wordlist
+   :members:
+
+`diceware.random_sources`
+-------------------------
+
+.. automodule:: diceware.random_sources
+   :members: