Rewrite README

[cdeil]

@michitaro - This is an attempt to improve the README, following #6.

• Extend the intro a bit
• Extend the install / use
• Add developer doc section
• Remove the long JS example using canvas from the front page, just link to the example.

It's written more from the perspective of people new TS / JS, like me, focused on step by step commands to get started.

Specifically for the "isntall and use with npm" section I'm not sure if the description is accurate. Is it the same for TS / JS projects? Is this line correct (I didn't try yet)?

import { healpix } from 'healpix';

Or is this * import like in your examples needed? I'm just familiar with Python, where * imports are discouraged.

[michitaro]

Thank you very much!

Specifically for the "isntall and use with npm" section I'm not sure if the description is accurate. Is it the same for TS / JS projects? Is this line correct (I didn't try yet)?

import { healpix } from 'healpix';

I made a tiny change on it