Docs on how to update mbgl dependency

[springmeyer]

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Currently to update mbgl one would need to:

• Package two new mbgl packages:
  • Release build (like https://github.com/mapbox/mason/pull/651)
  • Debug+ASAN build (like https://github.com/mapbox/mason/pull/652)
• Update the mbgl version in mason-versions.ini

This should be:

• [ ] documented in the readme
• [ ] and @springmeyer should spirit guide another developer through this process to ensure it is robust and we have redundency

[springmeyer]

👋 @zmofei - would you be interested in taking this on? 2 parts: a) updating to latest mbgl and b) documenting the process of doing it.

[zmofei]

👌 Sure, I'm happy to do it.