Document MSRV policy

README.md

@@ -115,6 +115,24 @@ A full list of all the unit tests we run in our CI can be found [here](.github/s
 MMTk is also tested with the VM bindings we are maintaining by running standard test/benchmark suites for the VMs.
 For details, please refer to each VM binding repository.
 
+## Supported Rust versions
+
+MMTk uses a pinned Rust version in the repository (recorded in the `rust-toolchain` file).  We run
+our tests and benchmarks using the pinned Rust version. We recommend using the pinned Rust version
+for development. We update the pinned Rust version between releases of mmtk-core to keep it close to
+the latest Rust stable release. The release cycle of mmtk-core is six weeks, roughly the same as
+Rust itself. 
+
+Our minimum support Rust version (MSRV) policy is "N-1" (note that N is *NOT* the current stable Rust release).  That means we also ensure mmtk-core works
+properly with the Rust toolchain that is one minor version before the version specified in
+`rust-toolchain`.  For example, if `rust-toolchain` contains "1.61.2", the MSRV will be guaranteed
+to be no later than "1.60.0".  We may bump MSRV up to "N-1" when we need to make use of new Rust
+features or a dependency crate that needs a newer Rust. In other words, we will not eagerly bump the MSRV, and the MSRV can lag behind "N-1" in practice. However, users shouldn't depend on this lag, and are encouraged to keep close to the
+latest Rust toolchain rather than relying on an outdated version of Rust.
+
+Note, however, that we may switch to a more conservative MSRV policy in the future when MMTk reaches
+a stable state.
+
 ## Contributing to MMTk
 
 Thank you for your interest in contributing to MMTk. We appreciate all the contributors. Generally you can contribute to MMTk by either