[WIP] Generate README.md automagically

[azerella]

🚧 Work in progress ! 🚧

This branch is based off of feature/hoist-node-modules-to-root. It features a script that will automatically generate the contents of the README.md for each component in the components/ folder.

UPDATE: 20/02/2019

• Added prop jsdoc descriptions for each singular component.
• Added missing defaultProp types for isRequired properties.

In automating the README.md I've found that certain components have multiple definitions inside. Automatically extracting these is very difficult and we will need to do a large refactor in order to resolve this. One suggestion might be to split up the sub-components into separate files.

Example:

- `callout` also contains a `calendar` component...
- `header` also contains a `headerBrand` component...
- `footer` also contains a `footerEnd` and `footerNav` component...

Another suggestion could be to publish these components separately.

Full-list of components to decouple:

- callout
- control-input
- footer
- header
- inpage-nav
- keyword-list
- main-nav
- progress-indicator
- select
- skip-link