Missing bits from pytest-qt 2.0 docs

[nicoddemus]

@The-Compiler

I'm a bit late, but I just realised check_params_cb and order aren't mentioned in the signal docs at all, and order isn't even in the changelog.

[nicoddemus]

Also add qmodeltester to README, under Features.

[MShekow]

I volunteer to write the check_param_cb and order documentation.

[nicoddemus]

Thanks @MShekow! We appreciate it. 😄