Docs link in building page

[choldgraf]

This PR adds a short sentence w/ a link to the Binder documentation from the loading page. Many BinderHub users (particularly new ones) will first experience Binder by clicking a link, not from going to mybinder.org. This makes sure that we show them a link to the docs on the loading page itself.

It should look like this when loading:

[betatim]

Suggested change

	<p class="text-center">New to Binder? Check out the <a target="_blank" href="https://mybinder.readthedocs.io/en/latest/">Binder Documentation</a> for more information</p>
	<p class="text-center">New to Binder? Check out the <a target="_blank" href="https://mybinder.readthedocs.io/en/latest/">Binder Documentation (opens in new window)</a> for more information</p>

What is the current UX recommendation about target=_blank? Personally I find it annoying when sites force me to open something in a new tab, as I am perfectly able to click "open in new tab" when I mean to do that. I can see that for new users it would be good to open it in a new window by default though maybe "new" in this context isn't new to binder but new to using browsers?

[choldgraf]

The main reason I did it here was because this is a temporary "loading" screen. So in a sense we know what users are expecting: they're expecting to be taken to a running Binder. But if they get redirected to a new page first, then that will never happen.

[betatim]

(restarted the build, it was a timeout error)

[choldgraf]

looks like travis is happy now, LMK what you think about the target=blank thing...I am not super opinionated on that one

[betatim]

Merged. We can tune this if we hear about people ranting about it online.