Adding docs on how to make a CAS server.

docs/source/data/cas-example-server.conf

+server:
+  - !channel
+    port: 50051
+    insecure_mode: true
+
+instances:
+  - name: main
+
+    storages:
+      - !disk-storage &main-storage
+	path: !expand-path $HOME/cas
+
+    services:
+      - !cas
+	storage: *main-storage
+      - !bytestream
+	storage: *main-storage
+      - !reference-cache
+	storage: *main-storage
+	max_cached_refs: 512

docs/source/using.rst

-
 .. _using:
 
 Using
@@ -12,3 +11,4 @@ This section covers how to run an use the BuildGrid build service.
    using_internal.rst
    using_bazel.rst
    using_buildstream.rst
+   using_cas_server.rst

docs/source/using_cas_server.rst

+.. _cas-server:
+
+CAS server
+==========
+
+It is possible to configure BuildGrid with just a Content Addressable Storage service.
+
+.. note::
+
+   This service can be equivalent to `BuildStream's Artifact Server`_ if the `Reference Storage Service`_ is included.
+
+.. _cas-configuration:
+
+Configuration
+-------------
+
+Here is an example project configuration. It also implements an optional API called the `Reference Storage Service`_, which if used, allows the user to store a ``Digest`` behind a user defined ``key``.
+
+.. literalinclude:: ./data/cas-example-server.conf
+   :language: yaml
+
+.. hint::
+
+   Use ``- name: ""`` if using with BuildStream, as instance names are not supported for that tool yet.
+
+This defines a single ``main`` instance of the ``CAS``, ``Bytestream`` and ``Reference Storage`` service on port ``55051``. It is backed onto disk storage and will populate the folder ``$HOME/cas``. To start the server, simply type into your terminal:
+
+.. code-block:: sh
+
+   bgd server start example.conf
+
+The server should now be available to use.
+
+.. _BuildStream's Artifact Server: https://buildstream.gitlab.io/buildstream/install_artifacts.html
+.. _Reference Storage Service: https://gitlab.com/BuildGrid/buildgrid/blob/master/buildgrid/_protos/buildstream/v2/buildstream.proto