Update the README.

README.md

@@ -1,4 +1,22 @@
 # mathdws
 ## A math workspace and libraries for Dyalog APL
 
-This repository 
+This repository contains a math workspace for Dyalog APL with functions for
+finding eigenvalues and eigenvectors, matrix inversion and division, and
+discrete Fourier transforms.
+
+These functions are implemented as calls into shared libraries. Source code and
+build scripts for the libraries are included.
+
+## Building the libraries
+
+The only supported build configuration is cross-compiling from Linux to Windows.
+
+To build (tested on Ubuntu 15.10):
+
+* install some cross-compilers with
+`sudo apt-get install gcc-mingw-w64 gfortran-mingw-w64`
+* `cd` into the root of the repository
+* type `make` (or e.g. `make -j8` if you have 8 CPU cores)
+
+This will generate the release package `math.zip` in the current directory.