added feedback gDoc

docs/contribute.rst

@@ -3,6 +3,7 @@ How to contribute
 
 Your feedback, esp. as a repository manager, is important for us. You can provide us feedback using the following channels:
 
+* by commenting our feedback document at https://docs.google.com/document/d/1aFWrkBO_f_GTWWTAfco4uoUphEbtOOdJVD_2ohYw7Ek/edit#bookmark=id.5suud9f7uqc2 (no account needed)
 * by creating an issue in our guidelines github repository (you need to have a github account): https://github.com/openaire/guidelines-literature-repositories/issues
 * by making annotations on the guidelines pages (you need to have an account for https://web.hypothes.is): just highlight some text and you can annotate the document
 * by sending an e-mail to: guidelines@openaire.eu