Create a checklist for adding new native build dependencies

[jdm]

I would like to see a wiki page that contains a checklist that needs to be addressed each time a pull request adds a build dependency that requires changes to the native build environment (most recently #21158 and #21029). Items on this checklist should include:

• updating the build setup instructions in README.md for all tier 1 platforms (windows, debian-based linuxes, mac)
• updating TravisCI and Appveyor configurations
• updating saltfs configuration
• updating configuration of nightly TravisCI servo build using rust nightly
• considering behaviour of build artifacts available on download.servo.org (c.f. #21414)

[Manishearth]

In general can we move from wiki pages to in-tree docs? They feel a bit more permanent.

Salt does this already (somewhat) and it's pretty nice.

I can write this when I get time.