add notes to README.md

Author: simonbyrne

README.md

@@ -5,6 +5,15 @@
 
 This is a Julia interface to libquadmath, providing a `Float128` type corresponding to the IEEE754 binary128 floating point format.
 
+## Support
+
+Quadmath currently works on x86_64 Linux and macOS.
+
+- It may require a new-ish version of gcc which supports `__float128` type.
+- It has not been tested on 32 bit Linux.
+- I have not had any luck getting it to work on Windows. It's probably something to do with the calling convention: if someone figures it out I would be very grateful.
+- It does not work on ARM due to the lack of libquadmath supportt for that platform.
+
 ## Installation
 ```julia
 Pkg.clone("https://github.com/simonbyrne/Quadmath.jl")