Fix install instructions for non-Docker

[yousefamar]

This project doesn't seem to be pure-node, and judging by the dependencies (as well as the Dockerfile) these packages at least need to be installed before npm install. Shouldn't this be added to the readme? And are there any other dependencies?

Also, npm install seems to fail under current node LTS potentially because of some upstream bug.

[yousefamar]

For anyone who might that issue I mentioned with canvas when you run npm run start -- -r, I got around it by removing whatever version is an internal dependency and reinstalling the latest version explicitly (adds "canvas": "^2.6.1", to package.json)