Add directions about re-reading state to coding standards

Author: nick-benoit

CODING_STANDARDS.md

@@ -36,6 +36,7 @@ This document outlines the coding standards and conventions used in the terrafor
 - Prefer using existing util functions over longer form, duplicated code:
   - `utils.IsKnown(val)` instead of `!val.IsNull() && !val.IsUnknown()`
   - `utils.ListTypeAs` instead of `val.ElementsAs` or similar for other collection types
+- The final state for a resource should be derived from a read request following a mutative request (eg create or update). We should not use the response from a mutative request to build the final resource state.
 
 ## Schema Definitions