Add note about NFS

Ran into problems with a JupyterHub installation on a cluster, had an awful time tracking this down. Possibly a rare occurrence, but it seems worth mentioning *somewhere*!

Author: ECoulter

README.md

@@ -194,6 +194,10 @@ error messages:
   batchspawner-specific, with the one exception below. The error log
   would be in the batch script output (same file as above). There may
   also be clues in the JupyterHub logfile.
+  
+- Are you running on an NFS filesystem? It's possible for Jupyter to 
+  experience issues due to varying implementations of the fcntl() system
+  call. (See also [Jupyterhub-Notes and Tips: SQLite](https://jupyterhub.readthedocs.io/en/latest/reference/database.html?highlight=NFS#sqlite))
 
 Common problems: