virtual environment policy

May I suggest clarifying what the installation procedure is when working with several environments?

Should ipympl be installed in the environment where jupyter resides or in the environment where it will be used?

Similar for the labextension install commands?

I have jupyter installed on a separate pyenv (based on kernel 3.7.5) that is made global.
When creating a new environment where I want to use `%pylab widget`, I need to run:

```bash
pyenv virtualenv 3.7.5 test-ipympl
pyenv activate test-ipympl
pip install ipykernel
ipython kernel install --use --name="test-ipympl"
pip install matplotlib
pip install ipympl
```

Then I need to refresh the browser and restart any kernel running on `test-ipympl` environment

Is this the correct way to do it?

Review the installation procedure, separate Jupyter and kernel environments, and labextension install commands described in the issue. Confirm the supported setup for ipympl and document the required commands and restart or refresh steps so users can install it in a separate environment without guesswork.

Written by the indexing model from the issue text.