Mention adding metrics-graphics

[armenzg]

If you just add react-metrics-graphics and try to use it as indicated in the README.md it won't work.

You will get this issue:

ERROR in ./node_modules/react-metrics-graphics/build/MetricsGraphics.js
Module not found: Error: Can't resolve 'metrics-graphics/dist/metricsgraphics.css' in '/Users/armenzg/repos/js-perf-dashboard/node_modules/react-metrics-graphics/build'

metrics-graphics is the peerDependency that is least likely to already be on a project, thus, it would be worth mention adding it to the docs.

[relistan]

@armenzg this saved me hours.

[wlach]

Thanks @armenzg for this report, not sure how I missed it! I'm reluctant to change anything about this repository right now, as it's out of sync with metrics-graphics as it stands and not maintained.