Add feedback request in README (elastic#1707)

Author: pquentin

README

@@ -24,6 +24,20 @@ Installation
 
   pip install elasticsearch-dsl
 
+Feedback 🗣️
+-----------
+
+The engineering team here at Elastic is looking for developers to participate in
+research and feedback sessions to learn more about how you use our Python client and
+what improvements we can make to their design and your workflow. If you're interested in
+sharing your insights into developer experience and language client design, please fill
+out this `short form`_. Depending on the number of responses we get, we may either
+contact you for a 1:1 conversation or a focus group with other developers who use the
+same client. Thank you in advance - your feedback is crucial to improving the user
+experience for all Elasticsearch developers!
+
+.. _short form: https://forms.gle/bYZwDQXijfhfwshn9
+
 Examples
 --------