Add npm install step to contributing doc (#217)
Add npm install step to contributing doc (#217)

Running `npm run welcome` without first installing `lerna` will result
in a number of errors.
reactotron/reactotron,infinitered/reactotron,infinitered/reactotron,rmevans9/reactotron,rmevans9/reactotron,rmevans9/reactotron,infinitered/reactotron,reactotron/reactotron,reactotron/reactotron,rmevans9/reactotron,infinitered/reactotron,reactotron/reactotron,reactotron/reactotron,rmevans9/reactotron