Document font formula contribution guidelines

[ronaldtse]

We need to elaborate these guidelines, and make it into a GitHub issue template for the Formulas repository.

Checks to be performed:

• Authoritative source
  • If not authoritative source, do more verification to ensure source is valid.
  • Download from authoritative repository, preferably from more than one mirror (same file, same hash). If authoritative repository does not exist, ensure there are multiple mirrors (and that those mirrors are legitimate)
• Font license is included in formula
• License status
  • If open licensed (SIL, CC, etc), done.
  • If source is an openly-licensed Linux distribution that is officially distributed, assume that license checks have been performed and that we point to it, done.
  • If proprietary licensed, check if it has the following restrictions
    • Platform restrictions (Windows, macOS?)
    • Installation restrictions