add README.md

README.md

+Nifty gridder
+=============
+
+Library for high-accuracy gridding/degridding of radio interferometry datasets
+
+(Highly experimental pre-release version!)
+
+Programming aspects
+- written in C++11, fully portable
+- shared-memory parallelization via OpenMP
+- Python interface available
+- kernel computation is performed on the fly, avoiding inaccuracies
+  due to table lookup and reducing overall memory bandwidth
+- gridding/degridding of a single w-plane is OpenMP-parallelized with good
+  scaling behaviour
+
+Numerical aspects
+- uses the analytical gridding kernel presented in
+  https://arxiv.org/abs/1808.06736
+- uses the "improved W-stacking method" described in
+  https://www.repository.cam.ac.uk/handle/1810/292298 (p. 139ff)
+- in combination these two aspects allow extremely accurate gridding/degridding
+  operations (L2 error compared to explicit DFTs can go below 1e-12) with
+  reasonable resource consumption