Readme example of bindings within celery tasks example, Support for channels v1.x

[mattyg]

Thanks so much for this project! It was extremely helpful and the missing piece I was searching for!

I updated the Readme to include how to get model bindings to work with model instances within celery tasks.

I also updated the Reame and requirements.txt to indicate support for only channels 1.x, until #42 is resolved.

[Herst]

Maybe this can then be the basis for the last proper release for channels 1.x.

[linuxlewis]

I would move the celery example out of the getting started. It's technically not required. I've fixed the package requirements on master