Add version info

[kito-cheng]

Toolchain folks are start to implement this, and as convention, we would like to ask a version in the document, I guess the b extension may go through the fast track without much accident if it's just alias of zba, zbb and zbs; anyway I think has version control should be easier for everyone :P

NOTE: Toolchain community folks are happy with any version other than 1.0, since that's RISC-V convention as we know - don't use 1.0 unless it's become frozen.

[ved-rivos]

Thanks. The release is generated with version set to 0.1.