Update polygon-protocols.md

Add PIP-18 link

Author: hsutaiyu

docs/innovation-design/polygon-protocols.md

@@ -14,7 +14,7 @@ Here is how the proposed development, with consensus of the Polygon community, w
 - Changes to network token.
 - Introduction of a Staking Layer, which will eventually serve all L2 chains in the Polygon network.
 
-PIP-18 has suggested a phased approach for upgrading Polygon PoS to a zkEVM validium. This community discussion is guiding the technical specification of the potential upgrade. If this PIP is accepted, Polygon PoS will eventually plug into the interop layer that will connect the entire Polygon network. 
+[PIP-18](https://forum.polygon.technology/t/pip-18-polygon-2-0-phase-0-frontier/12913) has suggested a phased approach for upgrading Polygon PoS to a zkEVM validium. This community discussion is guiding the technical specification of the potential upgrade. If this PIP is accepted, Polygon PoS will eventually plug into the interop layer that will connect the entire Polygon network. 
 
 Phase 0 of PIP-18 includes: