Add How-to on caching compressed content

[nisbet-hubbard]

This is an oft-asked question on various support forums, and the usual answer was: no, it can’t.

This PR supplies the missing tutorial.

[y82]

Hi @nisbet-hubbard,

Thank you for your contribution - we appreciate your work on the patch. However, as the brotli module is a third-party module outside of nginx codebase and maintenance, this change cannot be incorporated.

Please also consider the following guidelines:

• All changes should be combined into a single commit before opening a pull request
• Refer to the README for the guidelines on commit message formatting
• The initial doc revision is rev="1"

Thank you once again for your time and effort.

[nisbet-hubbard]

Closing in favour of #105.