Update release notes to say that lld/MinGW is production-ready.

llvm-svn: 341936

Author: rui314

lld/docs/ReleaseNotes.rst

@@ -13,11 +13,12 @@ Mach-O (macOS), MinGW and WebAssembly. lld is command-line-compatible with GNU
 linkers and Microsoft link.exe, and is significantly faster than these system
 default linkers.
 
-lld 7 for ELF and COFF are production-ready. lld/ELF can build the entire
+lld 7 for ELF, COFF and MinGW are production-ready. lld/ELF can build the entire
 FreeBSD/AMD64 and will be the default linker of the next version of the
 operating system. lld/COFF is being used to build popular large programs such as
-the Chrome web browser. Mach-O, MinGW and WebAssembly supports are still
-experimental.
+the Chrome web browser. lld/MinGW is being used by Firefox for their MinGW
+builds, though it needs a special-built sysroot. Mach-O and WebAssembly supports
+are still experimental.
 
 Non-comprehensive list of changes in this release
 =================================================