[README] Document MSRV bump policy

README.md

@@ -30,10 +30,15 @@ and additional building blocks and features are being added.
 
 We are trying to maintain a decent balance between being able to use older
 compiler versions and using new features that improve safety and usability
-of the crate.  We target 4 versions behind the latest stable Rust, which is
-about half a year (24 weeks) past.
+of the crate.  We will support the latest 4 versions of stable Rust, which is
+about half a year (24 weeks) past.  This means our MSRV is at least 4 versions
+behind the latest stable.
 
-### Minimum version: 1.80.1
+We will bump the MSRV when a dependency requires it (and we cannot support an
+older version of the dependency) or when we require new language features.  In
+either case, commits bumping the MSRV will document why it occurred.
+
+### Minimum version: 1.79.0
 
 The `rust-version` given in `Cargo.toml` is the oldest version that can
 be used to compile the crate with the minimal versions of all dependencies
@@ -42,7 +47,7 @@ selected.
 You can run `cargo +nightly update -Z minimal-versions` to ask Cargo to
 select these minimal versions for all dependencies.
 
-### Current version: 1.80.1
+### Current version: 1.79.0
 
 This is the minimum Rust version required to build with latest version of
 all dependencies at time of release. `Cargo.lock` contains these versions