Add a "Cobble is compatible with Android X.X+"in README.md

[reefsofts-rhosds]

Hello, I would like to know what is the minimum android version that cobble supports, or how to know it without building the app (my computer is not powerful enough to build and package an android app). For better user experience, i suggest adding a line in the README indicating the minimal version of android required.

[hellcp]

It's relatively easy to find that information in the code, here's the line that mentions the minimum SDK version: https://github.com/pebble-dev/mobile-app/blob/master/android/app/build.gradle.kts#L47 and here's the website that mentions which sdk version corresponds to which android version: https://apilevels.com/

[reefsofts-rhosds]

SDK is 29 i think and it corresponds to Android 10, right?