initial docs

docs/gettingstarted.rst

+=====
+Getting Started
+=====
+
+Overview
+--------------
+
+Following are three short tutorials to help new users get acquainted with how to create RailEnvs, how to train simple DQN agents on them, and how to customize them.
+
+To use flatland in a project::
+
+.. code-block:: python
+
+    import flatland