doc/README: Initial Readme Text

Fixes #11

README.rst

@@ -1,4 +1,17 @@
-Pybricks API
-=============
+Pybricks end-user API & Documentation
+=====================================
 
-Description
+This repository documents the Pybricks end-user MicroPython API.
+
+Each `Pybricks firmware`_ comes with the `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`_.
+See the `contributor's guide <CONTRIBUTING.md>`_ for acceptable changes and
+instructions to build the documentation locally.
+
+.. _Pybricks package: pybricks
+.. _Pybricks firmware: https://github.com/pybricks/pybricks-micropython
+.. _official documentation: https://docs.pybricks.com/