Added guide for updating translations to contributing guide

CONTRIBUTING.md

@@ -40,6 +40,36 @@ it is:
   the `master`, as it's always a matter of opinion whether if benefits the
   overall functionality of the project.
 
+### Missing translations?
+
+Material for MkDocs ships support for 50+ languages with the help of community
+contributions. When new features are added, sometimes, new translations are
+necessary as well. It's impossible for the maintainers of the project to update
+all translations (we just don't speak 50+ languages), so we have to rely on
+our contributors to incrementally update translations. This process is pretty
+simple, so if you find a translation missing in your language, follow these
+guidelines:
+
+1.  Fork the repository.
+
+2.  Open up the [translation file for your language] as well as the
+    [English translations], as they are always up-to-date. Compare them
+    side-by-side and add the missing translations. __Important__: only add the
+    translations that are different from the defaults, e.g. if your language
+    is left-to-right, don't add the `direction` translation, as English is
+    left-to-right as well. The following translations are for technical
+    purposes, and should not be updated, so if they're missing from your
+    language it's for good reason:
+
+    - `search.config.lang`
+    - `search.config.pipeline`
+    - `search.config.separator`
+
+1.  Create a PR (see below) with your changes.
+
+  [translation file for your language]: https://github.com/squidfunk/mkdocs-material/tree/master/src/partials/languages
+  [English translations]: https://github.com/squidfunk/mkdocs-material/tree/master/src/partials/languages/en.html
+
 ## Submission guidelines
 
 ### Submitting an issue