Update README.md per behavior of HMR
Update README.md per behavior of HMR

Note, this change only includes the README. 

No testing was done to confirm that current behavior is as described.

Most older installations have the source_entry_path under the source_path.

For example:
source_path: /app/javascript
source_entry_path:  /app/javascript/packs
rails/webpacker,rails/webpacker