Add mojo IDL documentation for RenamedFrom.

Change-Id: I3632284e3369266779f3443884238010d1139f20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453495
Auto-Submit: Erik Chen <erikchen@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#814504}

mojo/public/tools/bindings/README.md

@@ -712,6 +712,24 @@ values and will need to deal with them gracefully. See
 [C++ Versioning Considerations](/mojo/public/cpp/bindings/README.md#Versioning-Considerations)
 for details.
 
+### Renaming versioned structs
+It's possible to rename versioned structs by using the `[RenamedFrom]` attribute.
+RenamedFrom
+
+``` cpp
+module asdf.mojom;
+
+// Old version:
+[Stable]
+struct OldStruct {
+};
+
+// New version:
+[Stable, RenamedFrom="asdf.mojom.OldStruct"]
+struct NewStruct {
+};
+```
+
 ## Grammar Reference
 
 Below is the (BNF-ish) context-free grammar of the Mojom language: