First pass at an 'extension attributes' doc

Makefile

@@ -5,4 +5,4 @@ verify:
 	@# Use "-x" if you want to skip exernal links
 	@tools/verify-links.sh -v .
 	@echo Running the RFC2119 keyword checker:
-	@tools/verify-phrases.sh -v spec.md
+	@tools/verify-phrases.sh -v spec.md extensions.md

extensions.md

@@ -0,0 +1,23 @@
+# CloudEvents Extension Attributes
+
+The [CloudEvents specification](spec.md) defines a set of metadata attributes
+than can be used when tranforming a generic event into a CloudEvent.
+The list of attributes specified in that document represent the minimal set
+that the specification authors deemed most likely to be used in a majority of
+situations.
+
+This document defines some addition attributes that, while not as commonly
+used as the ones specified in the [CloudEvents specification](spec.md),
+could still benefit from being formally specified in the hopes of providing
+some degree of interoperabilty. This also allows for attributes to be
+defined in an experimental manner and tested prior to being considered for
+inclusion in the [CloudEvents specification](spec.md).
+
+Implementations of the [CloudEvents specification](spec.md) are not mandated
+to limit their use of extension attributes to just the ones specified in
+this document. The attributes defined in this document have no official
+standing and might be changed, or removed, at any time.
+
+## Extension Attributes
+
+None at this time.

spec.md

@@ -216,6 +216,8 @@ that contains both context and data).
   middleware might want to include and provides a place to test metadata before
   adding them to the CloudEvents specification. TBD - Determine a shorter
   prefix for this (e.g. OpenAPI uses “x-”)
+  See the [Extensions](extensions.md) document for a list of possible
+  properties.
 * Constraints:
   * OPTIONAL
   * If present, MUST contain at least one entry