Document how the MSRV may be upgraded

Author: eldruin

README.md

@@ -52,6 +52,7 @@ version and an `-alpha` version side by side in a HAL.
 
 - [How-to: add a new trait](docs/how-to-add-a-new-trait.md)
 - [Version policy](docs/version-policy.md)
+- [MSRV](docs/msrv.md)
 
 ## Implementations and drivers
 
@@ -73,6 +74,8 @@ on crates.io.
 This crate is guaranteed to compile on stable Rust 1.46 and up. It *might*
 compile with older versions but that may change in any new patch release.
 
+See [here](docs/msrv.md) for details on how the MSRV may be upgraded.
+
 ## License
 
 Licensed under either of

docs/msrv.md

@@ -0,0 +1,28 @@
+# Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on all stable Rust versions going back to
+the version stated as MSRV in the README. It *might* compile with even older versions but
+that may change in any new patch release.
+
+## How the MSRV will be upgraded
+
+For `embedded-hal`, we do not consider upgrading the MSRV a strictly breaking change as defined by
+[SemVer](https://semver.org).
+
+We follow these rules when upgrading it:
+
+- We will not update the MSRV on any patch release: \_.\_.*Z*.
+- We may upgrade the MSRV on any *major* or *minor* release: *X*.*Y*.\_.
+- MSRV upgrades will be clearly stated in the changelog.
+
+This applies both to `0._._` releases as well as `>=1._._` releases.
+
+For example:
+
+For a given `x.y.z` release, we may upgrade the MSRV on `x` and `y` releases but not on `z` releases.
+
+If your MSRV upgrade policy differs from this, you are advised to specify the
+`embedded-hal` dependency in your `Cargo.toml` accordingly.
+
+See the [Rust Embedded Working Group MSRV RFC](https://github.com/rust-embedded/wg/blob/master/rfcs/0523-msrv-2020.md)
+for more background information and reasoning.