Make a note about the MSRV (bevyengine#4274)

# Objective

- Users occasionally ask about the latest supported Rust version.
- Fixes bevyengine#288 (kinda).

## Solution

- We use "latest stable release".
- Document this information in the README.

README.md

@@ -15,6 +15,9 @@ Bevy is a refreshingly simple data-driven game engine built in Rust. It is free
 
 Bevy is still in the _very_ early stages of development. APIs can and will change (now is the time to make suggestions!). Important features are missing. Documentation is sparse. Please don't build any serious projects in Bevy unless you are prepared to be broken by API changes constantly.
 
+**MSRV:** Bevy relies heavily on improvements in the Rust language and compiler.
+As a result, the Minimum Supported Rust Version (MSRV) is "the latest stable release" of Rust.
+
 ## Design Goals
 
 * **Capable**: Offer a complete 2D and 3D feature set