Add compiler/platform support matrix to README.md

[alexreinking]

I was mildly surprised to learn that we don't support building Halide with Homebrew's GCC 10 on macOS. By my understanding, we support the following host compilers and platforms:

Platform	Compiler	Version
Windows	MSVC	14.1+ (VS 2017)
Windows	MSVC	14.2+ (VS 2019)
macOS	AppleClang	??
Linux	GCC	5.0+
Linux	Clang	9+

Maybe this should just be pasted in to the README?

[abadams]

That's surprising. What goes wrong with homebrew gcc?

[alexreinking]

For one, PATH_MAX isn't imported correctly in Utils.cpp (needs the <sys/syslimits.h> header). I didn't get farther than that since I'm using Homebrew-installed LLVM, which is compiled against libc++, rather than libstdc++ and I haven't yet figured out how to force GCC to use it.