README: clarify running instructions

[waldyrious]

By the way, running npm install fails due to the execution of the prepublish script, which calls the package script, which runs, among other things, the lint and test scripts, both of which are currently failing. I'd recommend getting rid of the prepublish entry in package.json:

taskgraph/package.json

Line 121 in 590ad93

"prepublish": "npm run package",

...just as a quick fix to make sure people can at least try this out locally.