Add basic README for documentation

Signed-off-by: Robert Winkler <rwinkler@antmicro.com>

README.rst

@@ -0,0 +1,30 @@
+===============
+Getting Started
+===============
+
+Building documentation
+----------------------
+
+Activate the virtual environment and install dependencies::
+
+    make env
+    source enter-env.sh
+
+Build the whole documentation::
+
+    make html
+
+For more options see::
+
+    make help
+
+Packages in virtual environment
+-------------------------------
+
+To install packages in conda environment you can use both
+``conda`` and ``pip``.
+
+Note that ``pip`` is installed in the conda environment which uses ``python3``,
+and is related to ``pip3`` **inside** the virtual environment,
+whereas invocation of ``pip3`` directly uses your **system** ``pip3`` instance,
+typically located in ``/usr/bin/pip3``