Add docs for building TMA, contributing to repo.

Test: n/a adding docs
Change-Id: I8fa496a0d4f8794b12b737f09d36bfe203065bae

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+# Car test apps
+
+This repository is only for car test applications.
+
+## Building
+
+If you are not contributing to the repo, you can clone the repo via `git clone sso://googleplex-android/platform/packages/apps/Car/tests --branch pi-car-dev --single-branch`. Otherwise, see [workstation setup](#workstation-setup).
+
+Install [Android Studio](go/install-android-studio). Then import the `tests` Gradle project into Android Studio.
+
+### TestMediaApp
+
+TestMediaApp should be one of the run configurations. The green Run button should build and install the app on your phone.
+
+To see TestMediaApp in Android Auto Projected:
+
+1. Open Android Auto on phone
+2. Click hamburger icon at top left -> Settings
+3. Scroll to Version at bottom and tap ~10 times to unlock Developer Mode
+4. Click kebab icon at top right -> Developer settings
+5. Scroll to bottom and enable "Unknown sources"
+6. Exit and re-open Android Auto
+7. TestMediaApp should now be visible (click headphones icon in phone app to see app picker)
+
+## Contributing
+
+### Workstation setup
+
+Install [repo](https://source.android.com/setup/build/downloading#installing-repo) command line tool. Then run:
+
+```
+sudo apt-get install gitk
+sudo apt-get install git-gui
+mkdir WORKING_DIRECTORY_FOR_GIT_REPO
+cd WORKING_DIRECTORY_FOR_GIT_REPO
+repo init -u persistent-https://googleplex-android.git.corp.google.com/platform/manifest -b pi-car-dev -g name:platform/tools/repohooks,name:platform/packages/apps/Car/tests --depth=1
+repo sync
+```
+
+### Making a change
+
+```
+repo start BRANCH_NAME .
+# Make some changes
+git gui &
+# Use GUI to create a CL. Check amend box to update a work-in-progress CL
+repo upload .
+```
+