Document changes to make for windows

ChangeLog

@@ -9,6 +9,8 @@ Features
      the platform layer.
    * Add an option to use macros instead of function pointers in the platform
      layer (helps get rid of unwanted references).
+   * Improved Makefiles for Windows targets by fixing library targets and making
+     cross-compilation easier (thanks to Alon Bar-Lev).
 
 Bugfix
    * Fix hardclock() (only used in the benchmarking program) with some

README.rst

@@ -35,6 +35,8 @@ In order to run the tests, enter::
 
     make check
 
+In order to build for a Windows platform, you should use WINDOWS_BUILD=1 if the target is Windows but the build environment is Unix-like (eg when cross-compiling, or compiling from an MSYS shell), and WINDOWS=1 if the build environment is a Windows shell.
+
 Depending on your platform, you might run into some issues. Please check the Makefiles in *library/*, *programs/* and *tests/* for options to manually add or remove for specific platforms. You can also check `the mbed TLS Knowledge Base <https://polarssl.org/kb>`_ for articles on your platform or issue.
 
 In case you find that you need to do something else as well, please let us know what, so we can add it to the KB.