readme.md: Soften "supported" language for limited configuration architectures

These architectures have seen varying degrees of support from the kernel
or LLVM side, meaning that getting bugs fixed has not always been easy.
Make it clear that these are supported on a "best effort" basis.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: nathanchance

readme.md

@@ -43,7 +43,8 @@ and are relatively well supported and tested:
 * x86
 
 The following architectures are targetable from both LLVM and the Linux kernel
-and are supported and tested in limited configurations:
+and are known to work in specific/limited configurations and supported on a
+best-effort basis:
 * mips
 * powerpc
 * riscv