Flesh out the docs for building the docs

Make these more user friendly and helpful, less knowledge assumed, but
still very terse.

docs/documenters/buildthedocs.rst

 Building the Docs Locally
 =========================
 
+This documentation is hosted on `readthedocs <https://docs.readthedocs.io/>`_,
+which is a documentation hosting site based on the
+`Sphinx <http://www.sphinx-doc.org/>`_ documentation tool,
+which in turn uses the `reStructuredText <http://www.sphinx-doc.org/en/stable/rest.html>`_
+markup language.
+
+This section will show you how to build and serve these docs locally, so that
+you can make changes to `the documentation source files <https://github.com/hypothesis/client/tree/master/docs>`_
+and preview those changes locally before sending a pull request.
+
 You will need:
 
-#. Git
-#. Python and Virtualenv
-#. Make
+#. `Git <https://git-scm.com/>`_
+#. `Python <https://www.python.org/>`_ 2.7 or 3.4+
+#. `Virtualenv <https://virtualenv.pypa.io>`_
+#. `Make <https://www.gnu.org/software/make/>`_
+
+Once you've installed those, follow these steps in a terminal window to build
+and serve the docs locally:
+
+#. Checkout the https://github.com/hypothesis/client repo and ``cd`` into it:
+   
+   .. code-block:: sh
+   
+      git clone https://github.com/hypothesis/client.git
+      cd client
+
+#. Create and activate a Python virtual environment for the client
+   documentation:
 
-To build the docs:
+   .. code-block:: sh
 
-#. ``git clone https://github.com/hypothesis/client.git``
-#. ``cd client``
-#. Create and activate a Python virtual environment.
-#. ``pip install -r requirements-dev.in``
-#. ``make docs``
+      virtualenv .venv
+      source .venv/bin/activate
+
+#. Install the Python modules needed to build and serve the docs:
+   
+   .. code-block:: sh
+
+      pip install -r requirements-dev.in
+
+#. Build and serve the docs:
+   
+   .. code-block:: sh
+   
+      make docs
 
 Now open http://localhost:8888 to view your local build of the docs.
-The built docs will automatically update when you save changes to the source
-files.
+As long as ``make docs`` is running the built docs will automatically update
+when you save changes to the source files.
+
+If you've closed your terminal window and then you want to build and serve the
+docs again in the future, you don't need to install everything again.
+You just need to ``cd`` into the ``client`` directory, activate the virtualenv
+(``source .venv/bin/activate``) and then run
+``make docs``.