Document the bindgen-cli MSRV (#2516)

Author: pvdrz

README.md

@@ -39,11 +39,15 @@ extern "C" {
 
 ## MSRV
 
-The minimum supported Rust version is **1.60.0**.
+The `bindgen` minimum supported Rust version is **1.60.0**.
+
+The `bindgen-cli` minimum supported Rust version is **1.64.0**.
 
 No MSRV bump policy has been established yet, so MSRV may increase in any release.
 
-The MSRV is the minimum Rust version that can be used to *compile* `bindgen`. However, `bindgen` can generate bindings that are compatible with Rust versions below the current MSRV.
+The MSRV is the minimum Rust version that can be used to *compile* each crate. However, `bindgen` and `bindgen-cli` can generate bindings that are compatible with Rust versions below the current MSRV.
+
+Most of the time, the `bindgen-cli` crate will have a more recent MSRV than `bindgen` as crates such as `clap` require it. 
 
 ## API Reference