add: key work language to template

When writing your SIMD, you should follow keyword usage as specified in rfc 2119 and rfc 8174

XXXX-template.md

@@ -39,6 +39,8 @@ to another Solana core contributor. The generally means:
 - Interaction with other features
 - Edge cases
 
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119](https://www.ietf.org/rfc/rfc2119.txt) and [RFC 8174](https://www.ietf.org/rfc/rfc8174.txt).
+
 ## Impact
 
 How will the implemented proposal impacts dapp developers, validators, and core contributors?