add testimonial

docs/index.md

@@ -11,3 +11,9 @@ basics, you can customise the examples according to your needs.
 
 I'd like to postulate that the material covered here gives you enough tools
 that you can address at least 99% of inefficient `map_elements` tasks.
+
+## What are people saying?
+
+**Nelson Griffiths**, Engineering & ML Lead at Double River Investments | Core Maintainer Functime
+
+> this was an awesome intro. I am no rust expert, though I have written a few plugins. And I learned quite a bit from this! Having my team read it now as well. Thanks for putting this together. I think more content like this for people who don’t know how to write optimal polars code on the rust side will be really useful for people like me who want to work on plugins!