Add deprecated warning to readme

Type: feature

The following has been addressed in the PR:

[ ] There is a related issue
[ ] All code has been formatted with prettier as per the readme code style guidelines
[ ] Unit or Functional tests are included in the PR

Description:

Themes have been moved to the dojo/widgets repository, https://github.com/dojo/widgets/pull/932. Add message to the readme for the deprecation.

dojo / themes