Update the grader protocol documentation GRADERS.md

[markkuriekkinen]

GRADERS.md is a bit outdated.

• Not all HTTP GET query parameters are listed there
  • Relevant code that sets parameters: https://github.com/apluslms/a-plus/blob/a932c5104504f3367e81448ca8a42584f88f27a1/exercise/exercise_models.py#L1019-L1035
• ExerciseWithAttachment is deprecated and nobody is supposed to use it.

In addition, there are these documents:

• https://apluslms.github.io/protocols/aplus-assess-v1/
• https://apluslms.github.io/protocols/

They include some details that have never been implemented so that can lead to confusion. At any rate, take a look and compare if there are significant differences and something clearly outdated.