Make microG's Weblate more discoverable, encourage translation and contribution

[lucasmz-dev]

It is not clear that microG is translatable via Weblate when someone looks at this GitHub page. Right now, someone has to either go to Weblate's main instance and search for microG and find it, or, look in commits and check that, and this is assuming the person knows there's a project in Weblate.
Inside the README, or some other documentation, there should be some indiciation on how to translate the project.

It would also be important to note, or fix, the fact that microG-UI-tools needs to be translated separately.

I feel like there's people who would help translate the project but don't because it's not as comfortable as using Weblate, and there's also things like suggestions for people that want to help with context but aren't so sure (when there's other contributors).