DOC: add post-deploy update instructions [skip ci]

CONTRIBUTING.md

@@ -55,3 +55,7 @@ To publish a new version on npm:
     npm publish               # as sanand0
 
     git checkout dev
+
+Once published:
+
+- Update the version on [gramex/apps/ui/package.json](https://code.gramener.com/cto/gramex/blob/master/gramex/apps/ui/package.json)