Merge pull request #44 from dthaler/non-greased

Add caveat about what greasing doesn't do

Author: tfpauly

draft-edm-protocol-greasing.md

@@ -283,6 +283,12 @@ above, as guidance to implementers on how to correctly process protocol elements
 Documents can also provide test vectors, when applicable, that include grease values
 to ensure they are processed correctly.
 
+One limitation of greasing is that it only exercises grease values. It does not check whether
+other values that are reserved for future use are correctly treated as ignored or errors.
+As such, ossification remains a possibility for non-grease values. The goal of greasing
+is to increase the chances of correct implementation, and there by reduce (but not
+eliminate) the possibility of ossification.
+
 # Deployment Considerations and Incentives for Greasing
 
 Greasing can be used as a tool to improve the active use of existing protocol