Update CONTRIBUTORS.md and CONTRIBUTING.md with calls for translations (

#5779)

* Update contributing.md with call for translations

Adding in a section to let (potential) contributors know that they can contribute translations to the project.

* Update CONTRIBUTORS.md

Add link to translation contributors page

* Update CONTRIBUTING.md

Edited as per @aduth's suggestion.

CONTRIBUTING.md

@@ -69,3 +69,13 @@ To add a new documentation page, you'll have to create a Markdown file in the [d
 ## Reporting Security Issues
 
 Please see [SECURITY.md](./SECURITY.md).
+
+## Localizing Gutenberg Plugin
+
+To translate Gutenburg in your locale or language, [select your locale here](https://translate.wordpress.org/projects/wp-plugins/gutenberg) and translate *Development* (which contains the plugin's string) and/or *Development Readme* (please translate what you see in the Details tab of the [plugin page](https://wordpress.org/plugins/gutenberg/)).
+
+A Global Translation Editor (GTE) or Project Translation Editor (PTE) with suitable rights will process your translations in due time.
+
+Language packs are automatically generated once 95% of the plugin's strings are translated and approved for a locale.
+
+The eventual inclusion of Gutenburg into WordPress core means that more than 51% of WordPress installations running a translated WordPress installation will have Gutenburg's translated strings compiled into the core language pack as well.

CONTRIBUTORS.md

@@ -2,7 +2,7 @@
 
 Gutenberg is built by many contributors and volunteers. Thanks to all of them for their work!
 
-This list is manually curated to include valuable contributions by volunteers that do not include code, such as user testing, providing feedback, or mockups. Please edit this list to include new contributors as they come in. There is no particular order to this list. If you or someone else were omitted from this list, we assure you that was not intentional. Please let us know and we'll add you.
+This list is manually curated to include valuable contributions by volunteers that do not include code, such as user testing, providing feedback, or mockups. Please edit this list to include new contributors as they come in. There is no particular order to this list. If you or someone else were omitted from this list, we assure you that was not intentional. Please let us know and we'll add you. For volunteers who contributed their translations, your names are listed on [WordPress Translate site here](https://translate.wordpress.org/projects/wp-plugins/gutenberg/contributors).
 
 | GitHub Username | WordPress.org Username|
 | --------------- | --------------------- |