Merge pull request #441 from jupyter/py2-readme

[ci skip] Note in README about Python 2.x

README.md

@@ -40,6 +40,7 @@ Starting with [git commit SHA 9bd33dcc8688](https://github.com/jupyter/docker-st
 
 ## Other Tips and Known Issues
 
+* Python 2.x was [removed from all images](https://github.com/jupyter/docker-stacks/pull/433) on August 10th, 2017, starting in tag `cc9feab481f7`. If you wish to continue using Python 2.x, pin to tag `82b978b3ceeb`.
 * `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.
@@ -98,4 +99,4 @@ Update the `ubuntu:16.04` SHA in the most-base images (e.g., base-notebook). Sub
 3. `docker login` with your Docker Hub/Cloud credentials.
 4. Run `make retry/release-all`.
 
-When `make retry/release-all` successfully pushes the last of its images to Docker Hub (currently `jupyter/all-spark-notebook`), Docker Hub invokes [the webhook](https://github.com/jupyter/docker-stacks/blob/master/internal/docker-stacks-webhook/) which updates the [Docker build history](https://github.com/jupyter/docker-stacks/wiki/Docker-build-history) wiki page.
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+When `make retry/release-all` successfully pushes the last of its images to Docker Hub (currently `jupyter/all-spark-notebook`), Docker Hub invokes [the webhook](https://github.com/jupyter/docker-stacks/blob/master/internal/docker-stacks-webhook/) which updates the [Docker build history](https://github.com/jupyter/docker-stacks/wiki/Docker-build-history) wiki page.