Merge pull request OAI#925 from RobDolinMS/patch-5

[Schema] Clarify that not required -> optional

Author: webron

versions/3.0.md

@@ -177,6 +177,8 @@ Relative references used in $ref are processed as per [JSON Reference](https://t
 
 ### Schema
 
+In the following description, if a field is not explicitly **Required** or described with a MUST or SHALL, it can be considered OPTIONAL. 
+
 #### <a name="oasObject"></a>OpenAPI Object
 
 This is the root document object for the API specification.