Document MSRV

Author: matklad

src/tools/rust-analyzer/lib/smol_str/README.md

@@ -20,3 +20,9 @@ Unlike `String`, however, `SmolStr` is immutable. The primary use case for
 languages. Strings consisting of a series of newlines, followed by a series of
 whitespace are a typical pattern in computer programs because of indentation.
 Note that a specialized interner might be a better solution for some use cases.
+
+## MSRV Policy
+
+Minimal Supported Rust Version: latest stable.
+
+Bumping MSRV is not considered a semver-breaking change.