Add info on installing with CRT support (#2812)
Add info on installing with CRT support (#2812)

* Add info about CRT support
This update adds a new section to the Quickstart article,
"Using the AWS Common Runtime (CRT)". This section covers
(briefly) what CRT is and why it matters, then what it's
currently used for, plus how to install and remove CRT
support from a project.
boto/boto3