v0.8.18 (Change version number from v1 to v0.8 according to the Specification Development Guidance)

According to the Specification Development Guidance (https://riscv.atlassian.net/wiki/spaces/TAXX/pages/174522378/RISC-V+Development+Process#Specification-Development), specifications in freezing phase should have a version number of 0.8x.

Historical releases (v0.9.6 to v1.0.0-rc5) were all in the freezing phase, which results in this release being v0.8.18.

What's Changed

• Update the specification to align with the freeze requirements. by Bicheng Yang (@ybc-alkaid) in #87
• update version number by Bicheng Yang (@ybc-alkaid) in #88

Full Changelog: v1.0.0-rc5...v0.8.18