add Pushpay as a public reference

Author: hjgraca

README.md

@@ -88,6 +88,7 @@ Knowing which companies are using this library is important to help prioritize t
 The following companies, among others, use Powertools:
 
 * [Caylent](https://caylent.com/)
+* [Pushpay](https://pushpay.com/)
 
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