Update node-sass troubleshooting information (#2206)

Probably obvious to people who are familiar with the tools, I wasn't
sure how to fix the `node-sass` error as described. Hopefully these
changes save some other folks some of the time it took me to dig into
the sassy issue.

Updates the docs to with two bits of information:

1. Guidelines to move any webpack-related packages from
   `devDependencies` to `dependencies`
2. The instructions to rebuild `node-sass` if needed

Was able to verify this works with Rails 6.0.0

Author: csexton

docs/troubleshooting.md

@@ -4,22 +4,36 @@
 
 1. Read the error message carefully. The error message will tell you the precise key value
    that is not matching what Webpack expects.
-2. Put a `debugger` statement in your Webpack configuration and run `bin/webpack --debug`. 
+2. Put a `debugger` statement in your Webpack configuration and run `bin/webpack --debug`.
    If you have a node debugger installed, you'll see the Chrome debugger for your webpack
-   config. For example, install the Chrome extension [NiM](https://chrome.google.com/webstore/detail/nodejs-v8-inspector-manag/gnhhdgbaldcilmgcpfddgdbkhjohddkj) and 
+   config. For example, install the Chrome extension [NiM](https://chrome.google.com/webstore/detail/nodejs-v8-inspector-manag/gnhhdgbaldcilmgcpfddgdbkhjohddkj) and
    set the option for the dev tools to open automatically. For more details on debugging,
    see the official [Webpack docs on debugging](https://webpack.js.org/contribute/debugging/#devtools)
 
 ## ENOENT: no such file or directory - node-sass
 
-*  If you get this error `ENOENT: no such file or directory - node-sass` on Heroku
-or elsewhere during `assets:precompile` or `bundle exec rails webpacker:compile`
-then you would need to rebuild node-sass. It's a bit of a weird error;
-basically, it can't find the `node-sass` binary.
-An easy solution is to create a postinstall hook - `npm rebuild node-sass` in
-`package.json` and that will ensure `node-sass` is rebuilt whenever
-you install any new modules.
+If you get the error `ENOENT: no such file or directory - node-sass` on deploy with
+`assets:precompile` or `bundle exec rails webpacker:compile` you may need to
+move Sass to production `dependencies`.
 
+Move any packages that related to Sass (e.g. `node-sass` or `sass-loader`) from
+`devDependencies` to `dependencies` in `package.json`. This is because
+webpacker is running on a production system with the Rails workflow to build
+the assets. Particurly on hosting providers that try to detect and do the right
+thing, like Heroku.
+
+However, if you get this on local development, or not during a deploy then you
+may need to rebuild `node-sass`. It's a bit of a weird error; basically, it
+can't find the `node-sass` binary.  An easy solution is to create a postinstall
+hook to ensure `node-sass` is rebuilt whenever new modules are installed.
+
+In `package.json`:
+
+```json
+"scripts": {
+  "postinstall": "npm rebuild node-sass"
+}
+```
 
 ## Can't find hello_react.js in manifest.json