Add rule for vendor CSR naming scheme

In the last LLVM sync up meeting, we have dicussed the vendor CSR
stuffs, and we realized that we have rule for vendor extenison name and assembly
mnemonic, but no rule for vendor CSR naming scheme yet.

So it's would be great we have one before anyone start to upstream their
own CSR definition.

The naming scheme basically follow the assembly mnemonic - use vendor
prefix, the only difference is use underscore rather than dot, because
it could be used in marco definition, but dot can't, that's an important
character since that allow user to use marco before assmbler support,
and eaiser to migrate to new version once assmbler support.

README.mkd

@@ -333,6 +333,15 @@ Vendors should also aim to follow the conventions used for naming mnemonics
 in the ratified base ISA and extensions (e.g. the use of 'w', 'd',
 'u', and 's' suffixes).
 
+### CSR naming scheme
+
+Vendors may define their own CSRs within the custom read-only CSR address range
+specified in the RISC-V ISA spec. However, to avoid conflicts, each vendor CSR
+must include a prefix corresponding to the vendor's name.
+
+The vendor prefix should match the prefix defined in assembly mnemonics and be
+separated by a dot, e.g., `th.vxrm`.
+
 ### List of vendor prefixes