Document context injection

[timvink]

Awesome project ! Love using it.

I just spend 15 minutes searching for how to add context injecting on the command line. It's documented in the README:

Inject extra context with command-line arguments:
cookiecutter --no-input gh:msabramo/cookiecutter-supervisor program_name=foobar startsecs=10

Adding that bit of info would fit on either of these two pages:

• https://cookiecutter.readthedocs.io/en/1.7.2/advanced/injecting_context.html
• https://cookiecutter.readthedocs.io/en/1.7.2/advanced/user_config.html

Happy to provide a PR if you want it :)

[jensens]

fixed with #1677