Improve README

[aluxian]

Add info about:

• channels
• docker
• badge

[danieltrolezi]

PLEASE provide more information about how to configure the server. Thanks.

[Zamiell]

Please add to the README file that, in the user's GitHub Releases section of their repository, they NEED to prefix their releases with a "v" character or squirrel-updates-server won't work.

Also, a note to the maintainer: This convention isn't necessarily standard. For example, look at the Atom Releases page: https://github.com/atom/atom/releases

Perhaps squirrel-updates-server should be intelligent enough to look for both "#.#.#" AND "v#.#.#" in the user's repository and match whichever it finds? Or, alternatively, it could be a configurable setting in the conf.js file?