Add a cartopy install snippet to external colab.
Add a cartopy install snippet to external colab.

We've seen external users get confused trying to install cartopy, since the
website doesn't really have install instructions, eg
https://github.com/googlecolab/colabtools/issues/85.

This CL just adds a snippet with install instructions, and adds it to the list
of imports we offer pointers for.

RELNOTES: n/a

Build log: https://console.cloud.google.com/gcr/builds/35109215-56ad-4dcd-8311-cb89dd6713f7?project=colab-datalab
Pushed docker image: gcr.io/colab-datalab/datalab:auto-188827433-2018-03-12T22-13-15-342061983-07-00

PiperOrigin-RevId: 188931623

googlecolab/colabtools,googlecolab/colabtools