[docs] Add Subheader to v0.x migration guide (#12144)

* [Subheader] Add SubHeader component to Migration From V0.x Document

* Update migration-v0x.md

Author: shabareesh

docs/src/pages/guides/migration-v0x/migration-v0x.md

@@ -119,6 +119,16 @@ This will apply a change such as the following:
 +<Button variant="contained" />
 ```
 
+### Subheader
+
+```diff
+-import Subheader from 'material-ui/Subheader';
++import ListSubheader from '@material-ui/core/ListSubheader';
+
+-<Subheader>Sub Heading</Subheader>
++<ListSubheader>Sub Heading</ListSubheader>
+```
+
 ### To be continued…
 
 Have you successfully migrated your app, and wish to help the community?