Admin API for creating new users (#3415)

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diff --git a/docs/admin_api/register_api.rst b/docs/admin_api/register_api.rst
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+Shared-Secret Registration
+==========================
+
+This API allows for the creation of users in an administrative and
+non-interactive way. This is generally used for bootstrapping a Synapse
+instance with administrator accounts.
+
+To authenticate yourself to the server, you will need both the shared secret
+(``registration_shared_secret`` in the homeserver configuration), and a
+one-time nonce. If the registration shared secret is not configured, this API
+is not enabled.
+
+To fetch the nonce, you need to request one from the API::
+
+  > GET /_matrix/client/r0/admin/register
+
+  < {"nonce": "thisisanonce"}
+
+Once you have the nonce, you can make a ``POST`` to the same URL with a JSON
+body containing the nonce, username, password, whether they are an admin
+(optional, False by default), and a HMAC digest of the content.
+
+As an example::
+
+  > POST /_matrix/client/r0/admin/register
+  > {
+     "nonce": "thisisanonce",
+     "username": "pepper_roni",
+     "password": "pizza",
+     "admin": true,
+     "mac": "mac_digest_here"
+    }
+
+  < {
+     "access_token": "token_here",
+     "user_id": "@pepper_roni@test",
+     "home_server": "test",
+     "device_id": "device_id_here"
+    }
+
+The MAC is the hex digest output of the HMAC-SHA1 algorithm, with the key being
+the shared secret and the content being the nonce, user, password, and either
+the string "admin" or "notadmin", each separated by NULs. For an example of
+generation in Python::
+
+  import hmac, hashlib
+
+  def generate_mac(nonce, user, password, admin=False):
+
+      mac = hmac.new(
+        key=shared_secret,
+        digestmod=hashlib.sha1,
+      )
+
+      mac.update(nonce.encode('utf8'))
+      mac.update(b"\x00")
+      mac.update(user.encode('utf8'))
+      mac.update(b"\x00")
+      mac.update(password.encode('utf8'))
+      mac.update(b"\x00")
+      mac.update(b"admin" if admin else b"notadmin")
+
+      return mac.hexdigest()