Add CodeTriage badge to square/assertj-android

Adds a badge showing the number of people helping this repo on CodeTriage.

[![Open Source Helpers](https://www.codetriage.com/square/assertj-android/badges/users.svg)](https://www.codetriage.com/square/assertj-android)

## What is CodeTriage?

CodeTriage is an Open Source app that is designed to make contributing to Open Source projects easier. It works by sending subscribers a few open issues in their inbox. If subscribers get busy, there is an algorithm that backs off issue load so they do not get overwhelmed

[Read more about the CodeTriage project](https://www.codetriage.com/what).

## Why am I getting this PR?

Your project was picked by the human, @schneems. They selected it from the projects submitted to https://www.codetriage.com and hand edited the PR. How did your project get added to [CodeTriage](https://www.codetriage.com/what)? Roughly over 1 year ago, [gauravsak](https://github.com/gauravsak) added this project to CodeTriage in order to start contributing. Since then, 2 people have subscribed to help this repo.

## What does adding a badge accomplish?

Adding a badge invites people to help contribute to your project. It also lets developers know that others are invested in the longterm success and maintainability of the project.

You can see an example of a CodeTriage badge on these popular OSS READMEs:

- [![Email clients like GMAIL do not render SVG images](https://www.codetriage.com/rails/rails/badges/users.svg)](https://www.codetriage.com/rails/rails) https://github.com/rails/rails
- [![Email clients like GMAIL do not render SVG images](https://www.codetriage.com/crystal-lang/crystal/badges/users.svg)](https://www.codetriage.com/crystal-lang/crystal) https://github.com/crystal-lang/crystal

## Have a question or comment?

While I am a bot, this PR was manually reviewed and monitored by a human - @schneems. My job is writing commit messages and handling PR logistics.

If you have any questions, you can reply back to this PR and they will be answered by @schneems. If you do not want a badge right now, no worries, close the PR, you will not hear from me again.

Thanks for making your project Open Source! Any feedback is greatly appreciated.