Add grade status badge

[markpatterson27]

Following on from https://education.github.community/t/autograder-score-on-readme/66289

Currently grade results are buried in the action output and are difficult for students to find. The proposal is to create a status badge of the grade which can be added to the README.

Status badges can be made using the points output from the audograding action, by customising the classroom.yml file.

Would it be possible though for the autograding action to create a grading status badge itself? (Therefore saving educators having to create their own classroom.yml file)