Add Pencil

README.md

@@ -386,7 +386,10 @@
 
 - [Atomic](https://atomic.io/) - Advanced interactive design features like logic, data, and variables, into the hands of every designer.
 
-- [Affinity Designer](https://affinity.serif.com/en-us/ui-design-software/) - The perfect tool for UI and UX design
+- [Affinity Designer](https://affinity.serif.com/en-us/ui-design-software/) - The perfect tool for UI and UX design.
+
+- [Pencil](http://pencil.evolus.vn/) - An open-source GUI prototyping tool that's available for ALL platforms.
+
 
 <sup>[back to table of contents](#user-content-contents)</sup>