Add authentication instructions to README

[greg-1-anderson]

Authentication options are discussed in #491 and #463; it would be good to have some of this information in the README as well.

[mislav]

Thanks for the suggestions. But, there are 2 reasons why I won't accept this PR right now:

1. The preferred way to authenticate right now isn't the GITHUB_TOKEN environment variable, since that requires people to generate the Personal Access Token and populate the variable manually. Instead, the preferred way to authenticate is by hub prompting you for username/password when it needs to for the first time. This method needs to documentation, since it happens by itself.
2. The next version of hub is going to change the authentication strategy to have a dedicated command to set up authentication Dedicated command for authentication & configuration #225. This means it will no longer prompt you interactively, but you will be required to set it up the first time (and additionally for every GitHub Enterprise host). When that ships, that command will have documentation of its own.

[greg-1-anderson]

The information on authenticating via GITHUB_TOKEN is not easy to find. Since this information is very interesting to folks setting up hub to work in CI environments, and will continue to be supported, is there some other document where these instructions could be recorded? https://hub.github.com or https://hub.github.com/hub.1.html, perhaps?

[mislav]

We added GITHUB_TOKEN docs to the man page in the 2.2-stable branch, but I forgot to publish it to the site. It's now published! Thanks for the reminder.