Adds the docs for client registration

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+.. MediaGoblin Documentation
+
+   Written in 2011, 2012 by MediaGoblin contributors
+
+   To the extent possible under law, the author(s) have dedicated all
+   copyright and related and neighboring rights to this software to
+   the public domain worldwide. This software is distributed without
+   any warranty.
+
+   You should have received a copy of the CC0 Public Domain
+   Dedication along with this software. If not, see
+   <http://creativecommons.org/publicdomain/zero/1.0/>.
+
+====================
+Registering a Client
+====================
+
+To use the GNU MediaGoblin API you need to use the dynamic client registration. This has been adapted from the `OpenID specification <https://openid.net/specs/openid-connect-registration-1_0.html>`_, this is the only part of OpenID that is being used to serve the purpose to provide the client registration which is used in OAuth. 
+
+The endpoint is ``/api/client/register``
+
+The parameters are:
+
+type
+    **required** - This must be either *client_associate* (for new registration) or *client_update*
+
+client_id
+    **update only** - This should only be used updating client information, this is the client_id given when you register
+
+client_secret
+    **update only** - This should only be used updating client information, this is the client_secret given when you register
+
+contacts
+    **optional** - This a space seporated list of email addresses to contact of people responsible for the client
+
+application_type
+    **required** - This is the type of client you are making, this must be either *web* or *native*
+
+application_name
+    **optional** - This is the name of your client
+
+logo_url
+    **optional** - This is a URL of the logo image for your client
+
+redirect_uri
+    **optional** - This is a space seporated list of pre-registered URLs for use at the Authorization Server
+
+
+Response
+--------
+
+You will get back a response::
+
+client_id
+    This identifies a client
+
+client_secret
+    This is the secret.
+
+expires_at
+    This is time that the client credentials expire. If this is 0 the client registration does not expire.
+
+=======
+Example
+=======
+
+Register Client
+---------------
+
+To register a client for the first time, this is the minimum you must supply::
+
+    {
+        "type": "client_associate",
+        "application_type": "native"
+    }
+
+A Response will look like::
+
+    {
+        "client_secret": "hJtfhaQzgKerlLVdaeRAgmbcstSOBLRfgOinMxBCHcb",
+        "expires_at": 0,
+        "client_id": "vwljdhUMhhNbdKizpjZlxv"
+    }
+
+
+Updating Client
+---------------
+
+Using the response we got above we can update the information and add new information we may have opted not to supply::
+
+    {
+        "type": "client_update",
+        "client_id": "vwljdhUMhhNbdKizpjZlxv",
+        "client_secret": "hJtfhaQzgKerlLVdaeRAgmbcstSOBLRfgOinMxBCHcb",
+        "application_type": "web",
+        "application_name": "MyClient!", 
+        "logo_url": "https://myclient.org/images/my_logo.png",
+        "contacts": "myemail@someprovider.com another_developer@provider.net",
+    }
+
+The response will just return back the client_id and client_secret you sent::
+
+    {
+        "client_id": "vwljdhUMhhNbdKizpjZlxv",
+        "client_secret": "hJtfhaQzgKerlLVdaeRAgmbcstSOBLRfgOinMxBCHcb",
+        "expires_at": 0
+    }
+
+
+======
+Errors
+======
+
+There are a number of errors you could get back, This explains what could cause some of them:
+
+Could not decode JSON
+    This is caused when you have an error in your JSON, you may want to use a JSON validator to ensure that your JSON is correct.
+
+Unknown Content-Type
+    You should sent a Content-Type header with when you make a request, this should be either application/json or www-form-urlencoded. This is caused when a unknown Content-Type is used.
+
+No registration type provided
+    This is when you leave out the ``type``. This should either be client_update or client_associate
+
+Unknown application_type.
+    This is when you have provided a ``type`` however this isn't one of the known types.
+
+client_id is required to update.
+    When you try and update you need to specify the client_id, this will be what you were given when you initially registered the client.
+
+client_secret is required to update.
+    When you try to update you need to specify the client_secrer, this will be what you were given when you initially register the client.
+
+Unauthorized.
+    This is when you are trying to update however the client_id and/or client_secret you have submitted are incorrect.
+
+Only set client_id for update.
+    This should only be given when you update.
+
+Only set client_secret for update.
+    This should only be given when you update.
+
+Logo URL <url> is not a valid URL
+    This is when the URL specified did not meet the validation.
+
+contacts must be a string of space-separated email addresses.
+    ``contacts`` should be a string (not a list), ensure each email is seporated by a space
+
+Email <email> is not a valid email
+    This is when you have submitted an invalid email address
+
+redirect_uris must be space-separated URLs.
+    ``redirect_uris`` should be a string (not a list), ensure each URL is seporated by a space
+
+URI <URI> is not a valid URI
+    This is when your URI is invalid.
+
+