[New Egg] SourceCoop

[redthirten]

Description

Adds a new Egg for SourceCoop: A cooperative server-side-only mod for multi-player Source Engine games (like Black Mesa), that enables players to play together on single-player campaigns or maps. This Egg currently only supports Black Mesa (the mod's primary game of focus) and it's official maps, but this may be expanded to more games in the future via additional Eggs. Please see the README for additional information.

Also, this PR adjusts the existing Black Mesa Egg's README to fix typos and make it more clear of the differences between that Egg and this one.

Sort-of fulfills @shodanx2's Egg request as a comment in this issue: pelican-eggs/eggs#1856

Checklist for all submissions

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• 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?

New egg Submissions

1. Does your submission pass tests (server is connectable)?
2. Does your egg use a custom docker image?
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
6. The egg was exported from the panel

[QuintenQVD0]

Looks all good to me