explain purpose in the readme

[bgoodwine]

• how to fork, modify, pull request
• how the test workflow works
• more detailed description of where things are
• link wiki
• systems student should be able to do it without prior knowledge (!!!)

[bgoodwine]

|-----------|
| DONE      |
|-----------|
(\__/) ||
(•ㅅ•) ||
/ 　 づ