Clean up known issues in README

* Mesos issue for https://github.com/jupyter/docker-stacks/issues/330
* Remove jpeg pinning note (it was unpinned a while ago)

README.md

@@ -43,7 +43,7 @@ Starting with [git commit SHA 9bd33dcc8688](https://github.com/jupyter/docker-st
 * `tini -- start-notebook.sh` is the default Docker entrypoint-plus-command in every notebook stack. If you plan to modify it in any way, be sure to check the *Notebook Options* section of your stack's README to understand the consequences.
 * Every notebook stack is compatible with [JupyterHub](https://jupyterhub.readthedocs.io) 0.5 or higher.  When running with JupyterHub, you must override the Docker run command to point to the [start-singleuser.sh](base-notebook/start-singleuser.sh) script, which starts a single-user instance of the Notebook server.  See each stack's README for instructions on running with JupyterHub.
 * Check the [Docker recipes wiki page](https://github.com/jupyter/docker-stacks/wiki/Docker-Recipes) attached to this project for information about extending and deploying the Docker images defined here. Add to the wiki if you have relevant information.
-* All stacks that derive from minimal-notebook have the conda jpeg package pinned to version 8 until https://github.com/jupyter/docker-stacks/issues/210 is resolved upstream.
+* The pyspark-notebook and all-spark-notebook stacks will fail to submit Spark jobs to a Mesos cluster when run on Mac OSX due to https://github.com/docker/for-mac/issues/68.
 
 ## Maintainer Workflow