readme: Remove a paragraph about the toolchain being immature

At this point, it is reasonably mature.

There still obviously are projects that won't build with llvm-mingw,
both due to unimplemented features that GCC/binutils have, or for
other reasons, but the toolchain has seen quite a lot of usage
since this paragraph originally was written.

README.md

@@ -91,10 +91,6 @@ Status
 The toolchain currently does support both C and C++, including support
 for exception handling.
 
-It is in practice new and hasn't been tested with quite as many projects
-as the regular GCC/binutils based toolchains yet. You might run into issues
-building non-trivial projects.
-
 
 Known issues
 ------------