docs: Describe what this tool do in short description

Hi, your project looks useful, but to understand what this tool is do i have to scroll readme for section https://github.com/google/wireit#dependencies

Please, save our time and add short description about tool to head of readme. Current description "Wireit upgrades your npm scripts to make them smarter and more efficient. " is give nothing info about what is this.

Let's simplify a search of this tool