Mention WoodpeckerCI in README (#728)

[WoodpeckerCI](https://woodpecker-ci.org) uses this lib, so mention it :)

Signed-off-by: 6543 <6543@obermui.de>

README.md

@@ -167,6 +167,7 @@ func main() {
 * [Melrōse](https://melrōse.org) uses Expr to implement its music programming language.
 * [Tork](https://www.tork.run/) integrates Expr into its workflow execution.
 * [Critical Moments](https://criticalmoments.io) uses Expr for it's mobile realtime conditional targeting system.
+* [WoodpeckerCI](https://woodpecker-ci.org) uses Expr for [filtering workflows/steps](https://woodpecker-ci.org/docs/usage/workflow-syntax#evaluate).
 
 [Add your company too](https://github.com/expr-lang/expr/edit/master/README.md)