Add a guideline for mixins (#9016)
Add a guideline for mixins (#9016)

* Add a guideline for mixins

* Apply suggestions from code review

Co-authored-by: Daniel D. Beck <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@ddbeck.com>

* Update docs/data-guidelines.md

Co-authored-by: Philip Jägenstedt <3ba3974e96044dcde818e6310ed799abe10671d4@foolip.org>

* Fix code block

Co-authored-by: Daniel D. Beck <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@ddbeck.com>
Co-authored-by: Philip Jägenstedt <3ba3974e96044dcde818e6310ed799abe10671d4@foolip.org>
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data