Documentation: add minimum clang/llvm version

Documentation: add minimum clang/llvm version

Based on a vote at the LLVM BoF at Plumbers 2020, we decided to start
small, supporting just one formal upstream release of LLVM for now.

We can probably widen the support window of supported versions over
time.  Also, note that LLVM's release process is different than GCC's.
GCC tends to have 1 major release per year while releasing minor updates
to the past 3 major versions.  LLVM tends to support one major release
and one minor release every six months.

Signed-off-by: Nick Desaulniers <[email protected]>
Tested-by: Gustavo A. R. Silva <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Reviewed-by: Masahiro Yamada <[email protected]>
Acked-by: Will Deacon <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>