Document platform support policy

README.md

@@ -54,6 +54,14 @@ crate features, WASM support and Custom RNGs see the
 
 This crate requires Rust 1.36.0 or later.
 
+## Platform Support
+
+This crate supports the same operating system or platform versions that the Rust standard library does, unless a
+[custom implementation](https://docs.rs/getrandom/latest/getrandom/#custom-implementations) is used instead.
+
+This means that as Rust drops support for old versions of operating systems (such as old Linux kernel versions, Android API levels, etc)
+in stable releases, `getrandom` may create new patch releases (`0.N.x`) that remove support for outdated platform versions.
+
 # License
 
 The `getrandom` library is distributed under either of