Add documentation of #58, selecting the port on the remote side

- Closes: #126

Author: rkdarst

README.md

@@ -19,6 +19,7 @@ This package formerly included WrapSpawner and ProfilesSpawner, which provide me
    ```python
       c = get_config()
       c.JupyterHub.spawner_class = 'batchspawner.TorqueSpawner'
+      import batchspawner.api
    ```
 3. Depending on the spawner, additional configuration will likely be needed.
 
@@ -52,6 +53,7 @@ to run Jupyter notebooks on an academic supercomputer cluster.
 
    ```python
    # Select the Torque backend and increase the timeout since batch jobs may take time to start
+   import batchspawner.api
    c.JupyterHub.spawner_class = 'batchspawner.TorqueSpawner'
    c.Spawner.http_timeout = 120
 
@@ -117,6 +119,7 @@ clusters, as well as an option to run a local notebook directly on the jupyterhu
 
    ```python
    # Same initial setup as the previous example
+   import batchspawner.api
    c.JupyterHub.spawner_class = 'wrapspawner.ProfilesSpawner'
    c.Spawner.http_timeout = 120
    #------------------------------------------------------------------------------
@@ -171,6 +174,10 @@ Added (developer)
 
 Changed
 
+* PR #58 changes logic of port selection, so that it is selected *after* the singleuser server starts.  This means that the port number has to be conveyed back to JupyterHub.  This requires the following changes:
+  - `jupyterhub_config.py` *must* explicitely import `batchspawner.api`
+  - If you override `Spawner.cmd`, note that the default command is now `batchspawner-singleuser`, not the default `jupyterhub-singleuser`.  This is to add a hook to report the port number back to the hub.
+  - If you have installed with `pip install -e`, you will have to re-install so that the new script `batchspawner-singleuser` is added to `$PATH`.
 * Update minimum requirements to JupyterHub 0.8.1 and Python 3.4.
 * Update Slurm batch script.  Now, the single-user notebook is run in a job step, with a wrapper of `srun`.  This may need to be removed using `req_srun=''` if you don't want environment variables limited.
 * Pass the environment dictionary to the queue and cancel commands as well.  This is mostly user environment, but may be useful to these commands as well in some cases. #108, #111  If these envioronment variables were used for authentication as an admin, be aware that there are pre-existing security issues because they may be passed to the user via the batch submit command, see #82.