Add `done` event

[daniel-bryant]

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See my original issue https://github.com/binaryminds/react-native-sse/issues/56 for some background.

Currently the library only supports close events, which fire when the connection is closed from the client side. An additional event for tracking server side closing of the connection would be useful. This PR adds a done event to do that. Luckily most of the plumbing is already there, so it was easy to hook up the new event. As far as naming goes, I just matched the state name like most other events seem to do. It still might be a little confusing, so I added some text to the README.

[EmilJunker]

LGTM 👍

[rafeautie]

any idea when this will merge? :)

[quentez]

@daniel-bryant Thank you for the great work. I hope this gets merged eventually, but in the meantime it 100% solved the problem we were having 🙇‍♂️

[daniel-bryant]

@quentez that is great to hear, happy to help!