docs: Add MSRV support policy (#74)

Author: keelerm84

README.md

@@ -44,3 +44,9 @@ working code.)
 
 Early stage release for feedback purposes. May contain bugs or performance
 issues. API subject to change.
+
+## Minimum Supported Rust Version
+
+This project aims to maintain compatibility with a Rust version that is at least six months old.
+
+Version updates may occur more frequently than the policy guideline states if external forces require it. For example, a CVE in a downstream dependency requiring an MSRV bump would be considered an acceptable reason to violate the six month guideline.