Document ability to change user-agent

[FezVrasta]

When you run isomorphic-fetch from node, you can define a custom User-Agent string. When you run it from a browser, you are not allowed to define a custom one.

I think this should be documented somewhere because it can lead to confusion.

[shenders13]

I find it hard to believe that set a User-Agent header from the Browser using fetch. Is that really the case? Does AJAX (or another library) allow you to set the User-Agent header from the browser?

[FezVrasta]

Nope, but since on node it works, it would be nice to document this difference between the two environments