Document requirements for third-party usage (#3867)

docs/index.rst

@@ -18,3 +18,4 @@ Contents
    topics/api/index
    topics/install/index
    topics/development/index
+   topics/third-party

docs/topics/third-party.rst

@@ -0,0 +1,19 @@
+.. _third-party:
+
+=================
+Third-Party Usage
+=================
+
+Running your own Add-ons server will likely require a few changes. There is currently no easy
+way to provide custom templates and since Firefox Accounts is used for authentication there is
+no way to authenticate a user outside of a Mozilla property.
+
+If you would like to run your own Add-ons server you may want to update addons-server to support
+custom templates and move the Firefox Accounts management to a `django authentication backend`_.
+
+Another option would be to add any APIs that you required and write a custom frontend. This work is
+already underway and should be completed at some point but help is always welcome. You can find
+the API work in this project and the fronend work in `addons-frontend`_.
+
+.. _django authentication backend: https://github.com/mozilla/addons-server/issues/3799
+.. _addons-frontend: https://github.com/mozilla/addons-frontend