add links to wiki auth/spawner lists

from the docs

docs/authenticators.md

@@ -11,6 +11,8 @@ One such example is using [GitHub OAuth][].
 Because the username is passed from the Authenticator to the Spawner,
 a custom Authenticator and Spawner are often used together.
 
+See a list of custom Authenticators [on the wiki](https://github.com/jupyter/jupyterhub/wiki/Authenticators).
+
 
 ## Basics of Authenticators
 

docs/howitworks.md

@@ -59,6 +59,8 @@ which regular users typically do not have
 
 [More info on custom Authenticators](authenticators.md).
 
+See a list of custom Authenticators [on the wiki](https://github.com/jupyter/jupyterhub/wiki/Authenticators).
+
 
 ### Spawning
 
@@ -72,4 +74,4 @@ and needs to be able to take three actions:
 
 [More info on custom Spawners](spawners.md).
 
-[An example using Docker](https://github.com/jupyter/dockerspawner).
+See a list of custom Spawners [on the wiki](https://github.com/jupyter/jupyterhub/wiki/Spawners).

docs/spawners.md

@@ -8,6 +8,9 @@ and a custom Spawner needs to be able to take three actions:
 2. poll whether the process is still running
 3. stop the process
 
+See a list of custom Spawners [on the wiki](https://github.com/jupyter/jupyterhub/wiki/Spawners).
+
+
 ## Spawner.start
 
 `Spawner.start` should start the single-user server for a single user.