Adds Stylelint (#18)

* Adds Stylelint

- New npm script, css-lint
- Pulls in Bootstrap's config
- Replaces named color with BS variable
- Updates readme with new script, also adds links to other tools

* Add css-lint script in `npm test`

* Mention Stylelint in the readme, add test to script table

* Mention npm test there too

* Explain stylelintignore and stylelintrc files

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

Author: mdo

.stylelintignore

@@ -0,0 +1,3 @@
+**/*.min.css
+**/dist/
+node_modules

.stylelintrc

@@ -0,0 +1,5 @@
+{
+  "extends": [
+    "stylelint-config-twbs-bootstrap/scss"
+  ]
+}

README.md

@@ -55,16 +55,18 @@ Open <http://localhost:3000> to see the page in action.
 
 ## Scripts
 
-The following npm scripts are available to you in this starter repo. With the exception of `npm start`, the remaining scripts can be run from your command line with `npm run scriptName`.
+The following npm scripts are available to you in this starter repo. With the exception of `npm start` and `npm test`, the remaining scripts can be run from your command line with `npm run scriptName`.
 
 | Script | Description |
 | --- | --- |
 | `server` | Starts a local server (<http://localhost:3000>) for development |
 | `watch` | Automatically recompiles CSS as it watches the `scss` directory for changes |
 | `css` | Runs `css-compile` and `css-prefix` |
 | `css-compile` | Compiles source Sass into CSS |
-| `css-prefix` | Runs Autoprefixer on the compiled CSS |
-| `css-purge` | Runs PurgeCSS to remove CSS that is unused by `index.html` |
+| `css-lint` | Runs [Stylelint](https://stylelint.io) against source Sass for code quality |
+| `css-prefix` | Runs [Autoprefixer](https://github.com/postcss/autoprefixer) on the compiled CSS |
+| `css-purge` | Runs [PurgeCSS](https://purgecss.com) to remove CSS that is unused by `index.html` |
+| `test` | Runs `css-lint` and `css`, in sequential order |
 
 ## Advanced usage
 
@@ -110,6 +112,12 @@ When your repository is generated, you won't see anything in the Actions tab unt
 
 [Learn more about GitHub Actions](https://github.com/features/actions), [read the Actions docs](https://help.github.com/en/actions), or [browse the Actions Marketplace](https://github.com/marketplace/actions).
 
+### Stylelint
+
+Stylelint is included, as is Bootstrap's default Stylelint config, [stylelint-config-twbs-bootstrap](https://github.com/twbs/stylelint-config-twbs-bootstrap). This is the same linter configuration we use in the main Bootstrap project. It's run via the `npm test` command, which is invoked in our `ci.yml` Actions workflow file.
+
+At the root of the repo, `.stylelintignore` is used to list files that we ignore when linting and `.stylelintrc` is where we tell Stylelint to use the Bootstrap config. The former is recommended based on your specific needs, while the latter is required.
+
 ## Copyright
 
 &copy; @mdo 2020-2021 and licensed MIT.