Import bromite custom-filters documentation

* We'll update this and find a better location for it too perhaps,
  but for now let's start with a quick copy / mirror
* https://www.bromite.org/custom-filters
* https://github.com/bromite/bromite.github.io/blob/f2022b51ccd5468f035e2acefe0024480ae2ac0c/custom-filters.md
* https://gitlab.com/CalyxOS/tmp/bromite-website/-/blob/f2022b51ccd5468f035e2acefe0024480ae2ac0c/custom-filters.md

Issue: calyxos#2679
Change-Id: Iee4dec5305e7b4e3eb5c51da9cea906ad597f885