add minimum version to readme

The minimum version is based on the version we explicitly test in CI, which is currently 1.21.0. It also outlines a policy for bumping the minimum version in minor releases if we need to. I think it's fair to say that we try hard not to bump that version unnecessarily, but I didn't want to get too defensive about it in the readme.

Author: KodrAus

README.md

@@ -13,6 +13,12 @@ as well as anything that requires non-const function calls to be computed.
 [![Documentation](https://docs.rs/lazy_static/badge.svg)](https://docs.rs/lazy_static)
 [![License](https://img.shields.io/crates/l/lazy_static.svg)](https://github.com/rust-lang-nursery/lazy-static.rs#license)
 
+## Minimum supported `rustc`
+
+`1.21.0+`
+
+This version is explicitly tested in CI and may only be bumped in new minor versions. Any changes to the supported minimum version will be called out in the release notes.
+
 
 # Getting Started