Document custom-lint task

[Barabas5532]

I can't find any documentation for the new custom-lint task.

It was causing the commit hook to fail with this error in my project:

Could not find package `custom_lint` or file `custom_lint`
  [EXC] "dart run custom_lint" failed with exit code 255
#0      ProgramRunner.stream (package:dart_pre_commit/src/util/program_runner.dart:91:11)
<asynchronous suspension>

I've managed to turn it off by adding

dart_pre_commit:
  custom-lint: false

to pubspec.yaml.

[Skycoder42]

Thanks for your report. This has been addressed with the latest release (5.2.0).

I added documentation for the task and also ensured it only runs if custom_lint is installed as dev dependency - as it makes no sense to run this task if they are not used. After updating, you should be able to remove the config again and the task should still not run unless you install custom_lint.