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

Author: justin808

README.md

@@ -204,7 +204,7 @@ Webpacker ships with two binstubs: `./bin/webpack` and `./bin/webpack-dev-server
 
 In development, Webpacker compiles on demand rather than upfront by default. This happens when you refer to any of the pack assets using the Webpacker helper methods. This means that you don't have to run any separate processes. Compilation errors are logged to the standard Rails log. However, this auto-compilation happens when a web request is made that requires an updated webpack build, not when files change. Thus, that can be painfully slow for front-end development in this default way. Instead, you should either run the `bin/webpack --watch` or run `./bin/webpack-dev-server`
 
-If you want to use live code reloading, or you have enough JavaScript that on-demand compilation is too slow, you'll need to run `./bin/webpack-dev-server` or `ruby ./bin/webpack-dev-server`. Windows users will need to run these commands in a terminal separate from `bundle exec rails s`. This process will watch for changes in the `app/packs/entrypoints/*.js` files and automatically reload the browser to match. This feature is also known as [Hot Module Replacement](https://webpack.js.org/concepts/hot-module-replacement/).
+If you want to use live code reloading, or you have enough JavaScript that on-demand compilation is too slow, you'll need to run `./bin/webpack-dev-server` or `ruby ./bin/webpack-dev-server`. Windows users will need to run these commands in a terminal separate from `bundle exec rails s`. This process will watch for changes in the relevant files, defined by `webpacker.yml` configuration settings for `source_path`, `source_entry_path`, and `additional_paths`, and it will then automatically reload the browser to match. This feature is also known as [Hot Module Replacement](https://webpack.js.org/concepts/hot-module-replacement/).
 
 ```bash
 # webpack dev server