Improve documentation for input type=file

[bvaughn]

This issue was originally reported by @kevinsimper via facebook/react/issues/3981

Nowhere on the react documentation is there a reference on how to handle <input type="file"/>

It is a speciel field that can not be serialized, and I think it deserve some kind of mention, maybe just a link to some other resource or if it is just like a normal input, just saying that.

I think it might be worth mentioning eg you can't actually programmatically set the value of a file input which is a pretty big deal- it means you're more susceptible to issues with reconciliation. You can only read. You also have to read from node.files which is just how those work even outside React, but still might be worth mentioning.

[vishalvrv9]

Hello is this issue still open and can I take this up? This is referenced to issue#218 (which is also open)
Sorry I am a little lost (new to open source contributions) but I think I can do this. Thanks.

[bvaughn]

This issue is all yours, @vishalvrv9! 😄

I've added an "in-progress" label so that others will know not to start work on the issue. If you change your mind about the issue, no worries! Just let me know so that I can remove the label and free it up for someone else to claim.

Cheers!

[bvaughn]

Resolved via #323

Thanks @vishalvrv9 !