add instruction for installing dependencies

README.md

@@ -14,7 +14,8 @@ To use this, you will need to create a `config.json` file of the form:
 }
 ```
 
-You can build from source by running `yarn build`.
+You can install the dependencies by running `yarn install` and build from
+source by running `yarn build`.
 
 Then you can run `yarn test <testname>`. (`testname` is currently ignored since
 there is only one test available), and follow the instructions and prompts.