Update README.md (#619)

Add SPAN Digital as an user of expr

README.md

@@ -162,6 +162,7 @@ func main() {
 * [Visually.io](https://visually.io) employs Expr as a business rule engine for its personalization targeting algorithm.
 * [Akvorado](https://github.com/akvorado/akvorado) utilizes Expr to classify exporters and interfaces in network flows.
 * [keda.sh](https://keda.sh) uses Expr to allow customization of its Kubernetes-based event-driven autoscaling.
+* [SPAN Digital](https://spandigital.com/) uses Expr in it's Knowledge Management products
 
 [Add your company too](https://github.com/expr-lang/expr/edit/master/README.md)