Add a MSRV policy (#229)

And exclude Orbital from it as it requires nightly anyway.

.github/workflows/ci.yml

@@ -49,6 +49,10 @@ jobs:
  - { target: x86_64-unknown-netbsd, os: ubuntu-latest, options: --no-default-features, features: "x11,x11-dlopen,wayland,wayland-dlopen" }
  - { target: x86_64-apple-darwin, os: macos-latest, }
  - { target: wasm32-unknown-unknown, os: ubuntu-latest, }
+ exclude:
+ # Orbital doesn't follow MSRV
+ - rust_version: '1.70.0'
+ platform: { target: x86_64-unknown-redox, os: ubuntu-latest }
  include:
  - rust_version: nightly
  platform: { target: wasm32-unknown-unknown, os: ubuntu-latest, options: "-Zbuild-std=panic_abort,std", rustflags: "-Ctarget-feature=+atomics,+bulk-memory" }
@@ -90,7 +94,6 @@ jobs:
  run: |
  cargo update -p ahash --precise 0.8.7
  cargo update -p bumpalo --precise 3.14.0
- cargo update -p redox_syscall@0.5.3 --precise 0.5.2
 
  - name: Build crate
  shell: bash

README.md

@@ -128,6 +128,31 @@ fn main() {
 }
 ```
 
+MSRV Policy
+==
+This crate's Minimum Supported Rust Version (MSRV) is **1.70**. Changes to
+the MSRV will be accompanied by a minor version bump.
+
+As a **tentative** policy, the upper bound of the MSRV is given by the following
+formula:
+
+```text
+min(sid, stable - 3)
+```
+
+Where `sid` is the current version of `rustc` provided by [Debian Sid], and
+`stable` is the latest stable version of Rust. This bound may be broken in case of a major ecosystem shift or a security vulnerability.
+
+[Debian Sid]: https://packages.debian.org/sid/rustc
+
+Orbital is not covered by this MSRV policy, as it requires a Rust nightly
+toolchain to compile.
+
+All crates in the [`rust-windowing`] organizations have the
+same MSRV policy.
+
+[`rust-windowing`]: https://github.com/rust-windowing
+
 Changelog
 ---------