Add description to package.json

[fregante]

If description is missing from package.json, the tools that use it (like $ npm info and npmhub) will unsuccessfully try to parse the readme and get nonsense.

Before

$ npm info @zeit/ncc

@zeit/ncc@0.22.3 | MIT | deps: none | versions: 77
[![CI Status](https://github.com/zeit/ncc/workflows/CI/badge.svg)](https://github.com/zeit/ncc/actions?workflow=CI) [![codecov](https://codecov.io/gh/zeit/ncc/branch/master/graph/badge.svg)](https://codecov.io/gh/zeit/ncc)
https://github.com/zeit/ncc#readme

bin: ncc

... etc

After

$ npm info @zeit/ncc

@zeit/ncc@0.22.3 | MIT | deps: none | versions: 77
Simple CLI for compiling a Node.js module into a single file, together with all its dependencies, gcc-style.
https://github.com/zeit/ncc#readme

bin: ncc

... etc

[styfle]

Thanks!