Document how to check and add language to lines of code counter (#502)

Author: ErikSchierboom

building/tooling/lines-of-code-counter.md

@@ -8,5 +8,33 @@ If the student submitted a test file, those will be ignored.
 While this works well for most submissions, some tracks might need to ignore additional files.
 For that, we'll allow the Lines of Code Counter to be [customized per track][lines-of-code-counter-customize].
 
+## Implementation
+
+We're using the [tokei][tokei] library to do the actual counting.
+As pull requests are often not merged quickly, we've created our own [fork][tokei-fork].
+This allows us to iterate more quickly and add support for any language we want.
+
+## Supported languages
+
+You can check to see if your language is supported out of the box by looking it up in the [languages.json][languages.json] file.
+
+### Adding new language
+
+If your language is **not** supported, you can add support by:
+
+1. Adding an entry for your language to the [languages.json] file.
+   Check the [language addition docs][adding-language] for more information.
+2. Adding tests to verify counting works correctly for your language.
+   Check the [adding tests docs][adding-tests] for more information.
+
+Once you've made these changes, open a pull request to the [exercism/tokei repository][tokei-fork].
+Here is an [example pull request][example-pr] that adds support for the Arturo language.
+
 [lines-of-code-counter]: https://github.com/exercism/lines-of-code-counter/
 [lines-of-code-counter-customize]: https://github.com/exercism/lines-of-code-counter/#ignore-additional-files
+[languages.json]: https://github.com/exercism/tokei/blob/master/languages.json
+[tokei]: https://github.com/XAMPPRocky/tokei
+[tokei-fork]: https://github.com/exercism/tokei
+[example-pr]: https://github.com/exercism/tokei/pull/14/files
+[adding-language]: https://github.com/exercism/tokei/blob/master/CONTRIBUTING.md#language-addition
+[adding-tests]: https://github.com/exercism/tokei/blob/master/CONTRIBUTING.md#tests