Adding a doc reference to cross-chain/arbitrum module

Author: dimpar

Diff for: README.adoc

@@ -46,7 +46,8 @@ This repository contains:
 - TBTC v2 link:system-tests/[System tests]
 - TBTC v2 link:docs/[documentation],
 - TBTC v2 Yearn vault link:yearn/[Solidity smart contracts],
-- TBTC v2 link:monitoring/[monitoring tool].
+- TBTC v2 link:monitoring/[monitoring tool],
+- TBTC v2 Cross chain link:cross-chain/arbitrum/[Arbitrum smart contracts].
 
 toc::[]