add README and add deprecation warnings to render_to_response

render_to_response is a janky util function with dead code that sets unused context variables on every call of it with a django builtin that has the same contract.

Removed it from these, but keeping here since other projects use it:

https://github.com/dimagi/core-hq/pull/558
https://github.com/dimagi/payments/pull/8
https://github.com/dimagi/hq-pact/pull/6
https://github.com/dimagi/phonelog/pull/5