message: docs: replace npm with yarn in lockfile readme
  PR Close
diff: All of our npm dependencies are locked via the `yarn.lock` file for the following reasons:
  - our project has lots of dependencies which update at unpredictable times, so it's important that
  - we update them explicitly once in a while rather than implicitly when any of us runs npm install
  -- locked dependencies allow us to reuse npm cache on travis, significantly speeding up our builds
  + we update them explicitly once in a while rather than implicitly when any of us runs `yarn install`
  +- locked dependencies allow us to reuse yarn cache on travis, significantly speeding up our builds
  (by 5 minutes or more)
  - locked dependencies allow us to detect when node_modules folder is out of date after a branch switch
  which allows us to build the project with the correct dependencies every time
  