Add src/ folder to npm package

[ukstv]

In other words, please, remove src/ from .npmignore. The folder absence leads to a lot of warnings, like

WARNING in ./~/@react-mdc/base/lib/index.js
(Emitted value instead of an instance of Error) Cannot find source file '../src/index.tsx': Error: Can't resolve '../src/index.tsx' in '/Users/ukstv/Documents/Pro/foo/node_modules/@react-mdc/base/lib'
 @ ./~/@react-mdc/typography/lib/Container.js 13:13-39
 @ ./~/@react-mdc/typography/lib/index.js
...