Editorial: Describe intentions for compatibility across implementations

Articulation of goals as ECMA 402 evolves, meant to address
Microsoft's concerns about making a separate, compatible implementation.

spec/overview.html

@@ -63,5 +63,22 @@ <h1>Implementation Dependencies</h1>
         </li>
       </ul>
     </p>
+
+    <emu-clause id="sec-compatibility">
+      <h1>Compatibility across implementations</h1>
+
+      <p>
+        ECMA 402 describes the schema of the data used by its functions. The
+        data contained inside is implementation-dependent, and expected to
+        change over time and vary between implementations. The variation is
+        visible by programmers, and it is possible to construct programs which
+        will depend on a particular output. However, this specification
+        attempts to describe reasonable constraints which will allow
+        well-written programs to function across implementations.
+        Implementations are encouraged to continue their efforts to harmonize
+        linguistic data.
+      </p>
+    </emu-clause>
   </emu-clause>
+
 </emu-clause>