doc: README mention MSRV and add badge

README.md

@@ -4,6 +4,7 @@
 [![MIT licensed][license-badge]](./LICENSE.md)
 [![Crate][crates-badge]][crates-url]
 [![docs.rs](https://img.shields.io/docsrs/statrs)][docs-url]
+![Crates.io MSRV](https://img.shields.io/crates/msrv/statrs)
 
 [actions-test-badge]: https://github.com/statrs-dev/statrs/actions/workflows/test.yml/badge.svg
 [crates-badge]: https://img.shields.io/crates/v/statrs.svg
@@ -99,3 +100,6 @@ test: Update statrs::distribution::Normal test_cdf
 
 Please allow at least one week before pinging issues/pr's.
 
+
+## MSRV Policy
+Not all of our dependencies have an MSRV policy, but our CI will check that our MSRV !(https://img.shields.io/crates/msrv/statrs) is upheld.