Missing README for unmock-browser

[ksaaskil]

To add:

• Add a note it intercepts fetch by overriding global.fetch
• Does not intercept XMLHttpRequest (yet)

To decide:

• Should users install unmock or unmock-browser? The first option would be simpler for documentation etc., but calling unmock.on() in development could then have undesired consequences (if the code runs also in the Node.js development server, unmock starts to intercept also there)
• Should users use unmock-browser or just unmock-fetch? The latter does not export a "stand-alone" unmock package like unmock-node, unmock-browser and unmock do, should it?