Added README for APK sizing

Author: allisonbm92

tools/apksize/README.md

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+# APK Size Tooling
+
+## Purpose
+
+This tooling measures the size of APKs using Firebase. The APKs are simple apps
+that exercise only a small faction of the API surface. These numbers help to
+show how an app's size might grow if Firebase is included.
+
+## How to Use
+
+There are two tasks defined in this subproject: generateMeasurements and
+uploadMeasurements. The former gathers the measurements and writes them to a
+file in the build directory. The latter is invoked by CI and uploads the report
+to an SQL database.
+
+The generateMeasurements task may be manually run with `./gradlew -q
+generateMeasurements -Ppull_request=999`. A pull request number is required to
+generate the report.
+
+## Current Support
+
+All projects in this repository are supported with an aggressive ProGuard
+profile. Less aggressive ProGuard profiles will be added at a future date.