readme: clearify that this project builds on stable Rust

... unless either `singleton` or `legacy` is used

README.md

@@ -29,7 +29,10 @@ in `riscv` dependency crate or RustSBI library itself.
 
 ## Minimum supported Rust version
 
-To compile this project, you may need at least nightly Rust version of `rustc 1.59.0-nightly (c5ecc1570 2021-12-15)`.
+To compile RustSBI library, you need at least stable Rust version of `rustc 1.64.0`.
+
+If you are using feature `singleton` or `legacy` to support legacy SBI extensions, you are required to install
+nightly Rust compiler. You may need at least nightly Rust version of `rustc 1.59.0-nightly (c5ecc1570 2021-12-15)`.
 
 ## Binary downloads