Move MSRV policy to its own section of README (#4616)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->

Making MSRV policy more clear per request in
#4615

----

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Author: landonxjames

README.md

@@ -13,7 +13,7 @@ Setup
 
 1. `./gradlew` will setup gradle for you. JDK 17 is required.
 2. Running tests requires a working Rust installation. See [Rust docs](https://www.rust-lang.org/learn/get-started) for
-installation instructions on your platform. The MSRV (**M**inimum **S**upported **R**ust **V**ersion) for the crates in this project is `stable-2`, i.e. the current `stable` Rust version and the prior two versions. Older versions may work.
+installation instructions on your platform.
 
 Development
 -----------
@@ -134,3 +134,6 @@ to generate more or less AWS service clients.
 ```
 
 The generated SDK will be placed in `aws/sdk/build/aws-sdk`.
+
+### MSRV Policy
+The MSRV (**M**inimum **S**upported **R**ust **V**ersion) for the crates in this project is `stable-2`, i.e. the current `stable` Rust version and the prior two versions. Older versions may work. In rare circumstances our MSRV may exceed `stable-2` for security purposes (ex: to update a dependency for a security fix that requires an MSRV bump).