Document more common pitfalls in the README

People are getting tripped up on these being unavailable in the polyfill.

I have enabled 2FA for my npm account, and thus CI is not allowed to publish releases on my behalf without my manual intervention.

field name can not by empty

Ref. #666 [ci skip]

Closes #661

The old one failed:
https://travis-ci.org/github/fetch/jobs/425715787

[ci skip]

Add flow definitions

* Mention module exports
* Expand on abortable functionality
* Remove Bower instructions since the project is defunct
* Remove Yarn instructions because they are a no-brainer

Closes #569

Match spec behavior re: unsupported body type

Update Karma and detect available browsers when testing

It seems like it's outdated:
https://travis-ci.org/github/fetch/builds/424048996

The polyfill used to throw an exception, but the spec dictates that
unsupported body types are simply converted to string.

Closes #576

This was only useful while debugging cache issues.

In Safari 11.1.2, the default for `credentials` is still "omit".

Adopt Contributor Covenant Code of Conduct

The Open Code of Conduct has been defunct for a while.

Change `credentials` default value to `same-origin`

https://github.com/whatwg/fetch/pull/585

Switch test suite from PhantomJS to Karma

https://travis-ci.org/github/fetch/builds/383771737
https://travis-ci.org/github/fetch/builds/383770926
https://travis-ci.org/github/fetch/builds/383767274
https://travis-ci.org/github/fetch/builds/383767224

https://nodejs.org/api/buffer.html#buffer_buffer_from_buffer_alloc_and_buffer_allocunsafe