Say that interfaces should use placeholder methods to fill all ordinals

This adds a short note to show how methods should be 'deleted' while
still keeping interface ordinals in the range [0, N-1]: Interfaces
should use placeholder method names.

Change-Id: I8bfb2ba589c194d0b7603727c392b2b32a9ce6f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2242691
Auto-Submit: Andrew Moylan <amoylan@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#792998}

mojo/public/tools/bindings/README.md

@@ -589,7 +589,9 @@ the following hard constraints:
     an ordinal value, all fields or methods must explicitly specify an ordinal
     value.
 * For an *N*-field struct or *N*-method interface, the set of explicitly
-    assigned ordinal values must be limited to the range *[0, N-1]*.
+    assigned ordinal values must be limited to the range *[0, N-1]*. Interfaces
+    should include placeholder methods to fill the ordinal positions of removed
+    methods (for example "Unused_Message_7@7()" or "RemovedMessage@42()", etc).
 
 You may reorder fields, but you must ensure that the ordinal values of existing
 fields remain unchanged. For example, the following struct remains