Docs - added mention to the fact that github webhook should be in json format

[TheZalRevolt]

Fixes Issue

This pr add to the documentation the fact that the github webhook should be in JSON format. This Closes #422 . If it's needed to add more info just let me know

Check List (Check all the applicable boxes)

• [x] I sign off on contributing this submission to open-source
• [x] My code follows the code style of this project.
• [x] My change requires changes to the documentation.
• [x] I have updated the documentation accordingly.
• [x] All new and existing tests passed.
• [x] This PR does not contain plagiarized content.
• [x] The title of my pull request is a short description of the requested changes.

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Note to reviewers