adds a readme

Author: renefritze

README.md

@@ -0,0 +1,23 @@
+Requirements
+============
+
+`deal.II` with development headers
+
+Usage
+=====
+
+```
+git clone --recurse-submodules https://github.com/pymor/pymor-deal.II
+cd pymor-deal.II
+python -m pip install pymor
+python -m pip install -e .
+python test/demo.py
+```
+
+
+Development notes
+=================
+
+We have a `pre-commit` config that is also run/checked in CI.
+Python source is formatted with `black`.
+C++ source is formatted with `clang-format`.