add README

README.md

+```
+This file is part of the dune-pybindx1 project:
+  https://github.com/dune-community/dune-pybindx1
+The copyright lies with the authors of this file (see below).
+License: BSD 2-Clause License (http://opensource.org/licenses/BSD-2-Clause)
+Authors:
+  Felix Schindler (2016)
+```
+
+This module wraps [pybind11](https://github.com/pybind/pybind11) as a
+DUNE module, to facilitate the build process with dunecontrol.
+
+To create Python bindings you should
+
+* define the cmake variable `DUNE_PYBINDXI_PYTHON_VERSION` (optional)
+* `include(DunePybindxiUtils)` in your CMakeLists.txt
+* call `dune_pybindxi_add_module(module)`, where module is one or several C++
+  source files
+
+In your sources, you may include the pybind11 header `pybind11/foo.h` as
+```
+#include <dune/pybindxi/foo.h>
+```
+
+For further documentation on pybind1 and Python bindings,
+[read the docs](http://pybind11.readthedocs.io/en/latest/).
+