Please include instructions on how to get started

**Is your feature request related to a problem? Please describe.**
I wanted to get started using the tool. Per https://developer.android.com/studio/command-line/bundletool#download I downloaded and unzipped the project. I didn't see a pre-compiled binary.

**Describe the solution you'd like**
Please include a few sentences in the README about how to actually get started with a "hello world" example.

**Describe alternatives you've considered**
I know "gradle" is used to build things, so I ran `./gradlew` included in the download to try to build the project myself. Unfortunately, it's not clear where the output is, if any.