androidenv: document update procedure

Atemu e48ca2fa 1ffba9f2

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pkgs/development/mobile/androidenv/README.md
··· 1 + # How to update 2 + 3 + 1. `./fetchrepo.sh` 4 + 2. `./mkrepo.sh` 5 + 3. Check the `repo.json` diff for new stable versions of `tools`, `platform-tools`, `build-tools`, `emulator` and/or `ndk` 6 + 4. Update the relevant argument defaults in `compose-android-packages.nix`