add README.md

interpol_ng/README.md

+interpol_ng
+===========
+
+Library for high-accuracy 4pi convolution on the sphere, which generates a
+total convolution data cube from a set of sky and beam `a_lm` and computes
+interpolated values for a given list of detector pointings.
+
+Algorithmic details:
+- the code uses `libsharp` SHTs to compute the data cube
+- shared-memory parallelization is provided via standard C++ threads.
+- for interpolation, the algorithm and kernel described in
+  https://arxiv.org/abs/1808.06736 are used. This allows very efficient
+  interpolation with user-adjustable accuracy.
+
+Installation and prerequisites:
+- execute `python3 setup.py install --user`. This requires the `g++` compiler.
+  If you want to compile with `clang`, use
+  `CC="clang -fsized-deallocation" python3 setup.py install --user`
+- `numpy` and `pybind11` are required for the Python interface.
+- For tests and demos, the package `pysharp` (also available in this repository)
+  is required.
+- For the unit tests, `pytest` is required.