docs: Add browser example for streaming files

[achingbrain]

Addresses #1053 by adding an example of how to use the streaming interface to add files sequentially and obtain the hash of the directory containing the files.

[daviddias]

HAAWT 🔥 Thanks @achingbrain :)

[daviddias]

Make sure to list it https://github.com/ipfs/js-ipfs/tree/master/examples#tutorials

[daviddias]

@achingbrain while at it, wanna try creating an example that shows how to use https://github.com/jhiesey/videostream to create a video stream from js-ipfs?

[achingbrain]

Sure, maybe on a different PR though 😉

[daviddias]

@achingbrain yeah, absolutely. Just add this example to the index as suggested above and it is good to merge :)

[mitra42]

Its something like that videostream example that we are looking for. What's missing is how to create a function such as in the https://github.com/jhiesey/videostream Readme that takes an opts structure with start and end, and retrieves those bytes from a IPFS stream.

[achingbrain]

Make sure to list it https://github.com/ipfs/js-ipfs/tree/master/examples#tutorials

Done!

[daviddias]

thank you :)