Warn about async rendering of Collections in README

[florianpilz]

We spent nearly an hour to find out that automatic re-rendering of Collections does indeed work. We took so long, since the same Jasmine code to assert the changed DOM did not work on collections. (We mocked the AJAX call with a sync callback, so we assumed nothing is async).

However, after looking in the code, rendering of a single model is done in sync and rendering of a collection is done async (using debounce of UnderscoreJS).

I think it would be good to at least warn in the README about this behaviour to spare others the wasted time. Better yet, both update mechanisms should work the same way.

[markijbema]

I agree, I think we shouldn'the make promises on updating synchronously (react doesn'the really do this either). A warning would help. I'm not sure on making model asynchronous as well. It would make this problem more clear, but it would make debugging harder (I think). But a PR for an improved readme is very welcome of course