Add some additional information to README.md

* Add more details about performance comparison to an emulator.
* Mention that Robolectric supports 15 different API levels.

README.md

@@ -2,7 +2,9 @@
 [![Build Status](https://github.com/robolectric/robolectric/actions/workflows/tests.yml/badge.svg)](https://github.com/robolectric/robolectric/actions?query=workflow%3Atests)
 [![GitHub release](https://img.shields.io/github/release/robolectric/robolectric.svg?maxAge=60)](https://github.com/robolectric/robolectric/releases)
 
-Robolectric is the industry-standard unit testing framework for Android. With Robolectric, your tests run in a simulated Android environment inside a JVM, without the overhead of an emulator.
+Robolectric is the industry-standard unit testing framework for Android. With Robolectric, your tests run in a simulated Android environment inside a JVM, without the overhead and flakiness of an emulator. Robolectric tests routinely run 10x faster than those on cold-started emulators.
+
+Robolectic supports running unit tests for *15* different versions of Android, ranging from Jelly Bean (API level 16) to S (API level 31).
 
 ## Usage