Merge "crafting-changes: Add some info on Optional/Nullable" into stable-3.1

Documentation/dev-crafting-changes.txt

@@ -159,6 +159,9 @@ run `./tools/setup_gjf.sh` to download a local copy and set up a
 wrapper script. If you run your own copy, please use the same version,
 as there may be slight differences between versions.
 
+[[code-rules]]
+== Code Rules
+=== Final
 When to use `final` modifier and when not (in new code):
 
 Always:
@@ -180,6 +183,12 @@ Never:
   be removed
   * method parameters: similar to local variables
 
+=== Optional / Nullable
+Recommended:
+
+  * Optionals in arguments are discouraged (use @Nullable instead)
+  * Return types should be objects or Optionals of objects, but not null/nullable
+
 [[code-organization]]
 == Code Organization