add README.md

README.md

+# Libsharp
+
+Library for efficient spherical harmonic transforms at arbitrary spins,
+supporting CPU vectorization, OpenMP and MPI.
+
+## Paper
+
+https://arxiv.org/abs/1303.4945
+
+## News
+
+### January 2019
+
+This update features significant speedups thanks to important algorithmic
+discoveries by Keiichi Ishioka
+(https://www.jstage.jst.go.jp/article/jmsj/96/2/96_2018-019/_article and
+personal communication).
+
+These improvements reduce the fraction of CPU time spent on evaluating the
+recurrences for Y_lm coefficients, which means that computing multiple
+simultaneous SHTs no longer have a big performance advantage compared to SHTs
+done one after the other.
+As a consequence, libsharp support for simultaneous SHTs was dropped, making
+its interface much simpler.
+
+With the proper compilers and flags (see the file COMPILE for details) libsharp
+is now built with support for SSE2, AVX, AVX2, FMA3, FMA4 and AVX512f and the
+appropriate implementation is selected dynamically at runtime. This should
+provide a very significant performance boost for everyone using pre-compiled
+portable binaries.
+
+### Compilation
+
+The library uses the standard `autotools` mechanism for configuration,
+compilation and installation. See the file `COMPILE` for configuration hints.