Add a known issue about the fact that sanitizers only are supported on x86

README.md

@@ -82,6 +82,7 @@ normal GCC/binutils based MinGW.
 - Address Sanitizer doesn't produce working backtraces for i686. Address
   Sanitizer requires using a PDB file for symbolizing the error location and
   backtraces.
+- The sanitizers are only supported on x86.
 - llvm-objcopy doesn't support COFF yet, so the docker images bundle binutils
   objcopy/strip for this purpose instead, with a wrapper to relabel ARM/ARM64
   binaries into something that binutils objcopy/strip accepts to work on,