doc/README: Initial Readme Text

Fixes #11

README.rst

@@ -1,4 +1,16 @@
-Pybricks API
-=============
+Pybricks end-user API & Documentation
+=====================================
+
+This repository documents the Pybricks end-user MicroPython API.
+
+Each [Pybricks firmware](https://github.com/pybricks/pybricks-micropython)
+comes with the [pybricks](pybricks) package. All modules, classes, methods,
+and functions in that package have optimized implementations in C for high
+performance and reduced memory footprint.
+
+This repository provides an empty, pure Python blueprint of that package. It is
+used to generate the [official documentation](https://docs.pybricks.com/).
+
+See the [contributor's guide](CONTRIBUTING.md) for acceptable changes and
+instructions to build the documentation locally.
 
-Description