update instructions to use `npx` rather than `install -g`

[Pomax]

npx has been part of npm as of v5.2, in mid 2017. and will run any tool that is not installed globally by just fetching it and then running it if not installed locally, or just "running it from the local node_modules" if it is installed, finally making global installs entirely unnecessary.

It would be a good idea to update the instructions to recommend using npx deoptigate app.js without telling folks to install it first, and instead adding a note that npx will prefer deoptigate from the local node module directory if it's been installed as local project dependency.