Mention the lack of llvm-objcopy for COFF as a known issue in the readme

README.md

@@ -82,6 +82,10 @@ 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.
+- 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,
+  and back.
 
 Additionally, one may run into other minor differences between GCC and clang.