Updated MinGW cross-compilation documentation.

CMake/README.txt

@@ -1,17 +1,16 @@
-This directory contains a collection of toolchain definitions for cross
-compilation, currently limited to compiling Win32 binaries on Linux.
-
-The toolchain file naming scheme is as follows:
-
-  host-system-compiler.cmake
+This directory contains a collection of toolchain definitions for
+cross-compiling for Windows using MinGW on various other systems.
 
 To use these files you add a special parameter when configuring the source tree:
 
   cmake -DCMAKE_TOOLCHAIN_FILE=<toolchain-file> .
 
-For example, to use the Debian GNU/Linux MinGW package, run CMake like this:
+The exact file to use depends on the prefix used by the MinGW binaries on your
+system.  You can usually see this in the /usr directory, i.e. the Ubuntu
+MinGW-w64 packages have /usr/x86_64-w64-mingw32 for the 64-bit compilers, so the
+correct invocation would be:
 
-  cmake -DCMAKE_TOOLCHAIN_FILE=CMake/i586-mingw32msvc.cmake .
+  cmake -DCMAKE_TOOLCHAIN_FILE=CMake/x86_64-w64-mingw32.cmake .
 
 For more details see this article: