complete creators guidelines with relator terms

Author: davorpa

docs/CONTRIBUTING.md

@@ -178,7 +178,17 @@ Our lists provide a minimal set of metadata: titles, URLs, creators, platforms,
 ##### Creators
 
 - We want to credit the creators of free resources where appropriate, including translators!
-- For translated works the original author should be credited.
+- The next formatting syntax can serve to give them this kind of relator roles:
+
+    ```markdown
+    * [A Translated Book](http://example.com/book.html) - John Doe, `trl.:` Mike The Translator
+    ```
+
+    where `trl.:` could be any abbreviation listed in <https://loc.gov/marc/relators/relaterm.html>.
+- For translated works, the original authors should be credited at the very beginning of the creators list.
+- We don't need relator terms for original authors.
+- Use a comma `,` to delimit each item inside the author list.
+- You can shorten author lists with "`et al.`".
 - We do not permit links for Creators.
 - For compilation or remixed works, the "creator" may need a description. For example, "GoalKicker" or "RIP Tutorial" books are credited as "`Compiled from StackOverflow documentation`".