Added Garry's Mod to README

[Ammar-XD]

Description

Checklist for all submissions

• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Have you tested and reviewed your changes with confidence that everything works?
• [ ] Did you branch your changes and PR from that branch and not from your master branch?
  • If not, why?:

• [x] You verify that the start command applied does not use a shell script
  • [ ] If some script is needed then it is part of a current yolk or a PR to add one
• [x] The egg was exported from the panel

New egg Submissions

1. [ ] Does your submission pass tests (server is connectable)?
2. [ ] Does your egg use a custom docker image?
   • [ ] Have you tried to use a generic image?
   • [ ] Did you PR the necessary changes to make it work?
3. [ ] Have you added the egg to the main README.md and any other README files in subdirectories of the egg (e.g /game_eggs) according to the alphabetical order?
4. [ ] Have you added a unique README.md for the egg you are adding according to the alphabetical order?
5. [ ] You verify that the start command applied does not use a shell script
   • [ ] If some script is needed then it is part of a current yolk or a PR to add one
6. [ ] The egg was exported from the panel