Add reference to RFC2119 (MUST/SHOULD/MAY).

Hardware-API.md

@@ -13,6 +13,7 @@ The existing pyb module already provides such abstraction but it is not as clean
 
 * Port - when used in this document, refers to some kind of hardware port.
 * Software - MicroPython ported to a particular CPU/MCU/SoC. A separate word used to disambiguate word "port" (which otherwise may refer to a hardware or software port)
+* MUST, SHOULD, MAY - Have meanings based on https://www.ietf.org/rfc/rfc2119.txt
 
 # Use cases