README missing instructions on how to execute bundletool

There's no step-by-step information contained within the README to go from cloning the tool to making use of it's functionality

There's resources which tell us what command to use (like the one below) but if we can't run `bundletool` in the first place then these instructions are pointless

`bundletool build-apks --bundle=/MyApp/my_app.aab --output=/MyApp/my_app.apks`

At the very least this question below along with an answer needs to be added to the README

**Q: After cloning the repo how do I execute the `bundletool` tool?**