Be explicit about what the specversion string is

We never actually said if its 0.1 or v0.1 or something else.

Signed-off-by: Doug Davis <dug@us.ibm.com>

Author: Doug Davis

spec.md

@@ -186,7 +186,9 @@ help intermediate gateways determine how to route the events.
 ### specversion
 * Type: `String`
 * Description: The version of the CloudEvents specification which the event
-  uses. This enables the interpretation of the context.
+  uses. This enables the interpretation of the context. Compliant event 
+  producers MUST use a value of `0.1` when referring to this version of
+  the specification.
 * Constraints:
   * REQUIRED
   * MUST be a non-empty string