docs/design: update the template of RFC docs (#14582)

docs/design/TEMPLATE.md

@@ -48,12 +48,17 @@ A discussion of alternate approaches and the trade-offs, advantages, and disadva
 - What is the impact of not doing this?
 -->
 
-## Compatibility
+## Compatibility and Mirgration Plan
 
 <!--
 A discussion of the change with regard to the compatibility issues:
 - Does this proposal make TiDB not compatible with the old versions?
+- If the existing behavior will be changed, how will we phase out the older behavior?
 - Does this proposal make TiDB more compatible with MySQL?
+- What is the impact(if any) on the data migration:
+    + from MySQL to TiDB
+    + from TiDB to MySQL
+    + from old TiDB cluster to new TiDB cluster
 -->
 
 ## Implementation
@@ -66,6 +71,14 @@ A detailed description for each step in the implementation:
 - How long it takes to accomplish it?
 -->
 
+## Testing Plan
+
+<!--
+A brief description on how the implementation will be tested. Both integration test and unit test should consider the following things:
+- How to ensure that the implementation works as expected?
+- How will we know nothing broke?
+-->
+
 ## Open issues (if applicable)
 
 <!--