small readme update

packages/contracts/README.md

@@ -2,6 +2,10 @@
 
 This package contains the various Ethereum smart contracts that make up the Layer 1 component of Optimism's Optimistic Rollup construction.
 
+## Building and Running
+
+This package requires that `yarn` be installed on your machine!  Once it is, run `yarn test` to build and run tests.
+
 ## Disclaimer
 
 The contracts in this package have **not** been audited. We **do not recommend** deploying these contracts in a production capacity.