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Since there's no architectural feature detection on RISC-V (unlike `CPUID` on x86 architectures and some system registers on Arm/AArch64), runtime feature detection entirely depends on the platform-specific facility. As a result, availability of each feature heavily depends on the platform and its version. To help users make a decision for feature checking on a RISC-V system, this commit adds a platform guide with minimum supported platform versions. Note: It intentionally omits the description of the reverse implication related to *extension groups* (such like implication of `B` *from* its members: `Zba`, `Zbb` and `Zbs` extensions) because it currently does not synchronize well with the `-Ctarget-feature` compiler option (due to missing reverse implication checks using `cfg` and due to constraints of the current Rust's feature handling). Instead, it only describes forward implications (like `D` implying `F`) due to the fact that it relatively synchronizes well between Rust and `stdarch` for this kind of feature handling (not fully synchronized though). Still, an extension group is considered "supported" once the platform/version supports runtime detection of all members in it.