Add a note about supported compilers
Adapted from the section in Mbed Crypto, with the addition of specific
compiler versions. Only mention Python 3; Python 2 may still work but
it is no longer supported upstream so we will not maintain
compatibility anymore.
diff --git a/README.md b/README.md
index c6dfd37..69200f2 100644
--- a/README.md
+++ b/README.md
@@ -40,6 +40,15 @@
The Make and CMake build systems create three libraries: libmbedcrypto, libmbedx509, and libmbedtls. Note that libmbedtls depends on libmbedx509 and libmbedcrypto, and libmbedx509 depends on libmbedcrypto. As a result, some linkers will expect flags to be in a specific order, for example the GNU linker wants `-lmbedtls -lmbedx509 -lmbedcrypto`. Also, when loading shared libraries using dlopen(), you'll need to load libmbedcrypto first, then libmbedx509, before you can load libmbedtls.
+### Tool versions
+
+You need the following tools to build the library with the provided makefiles:
+
+* GNU Make or a build tool that CMake supports.
+* A C99 toolchain (compiler, linker, archiver). We actively test with GCC 5.4, Clang 3.8, IAR8 and Visual Studio 2013. More recent versions should work. Slightly older versions may work.
+* Python 3 to generate the test code.
+* Perl to run the tests.
+
### Make
We require GNU Make. To build the library and the sample programs, GNU Make and a C compiler are sufficient. Some of the more advanced build targets require some Unix/Linux tools.