GitHub supports emoji (絵文字), the Japanese term for the picture characters or emoticons used in Japanese electronic messages and webpages.
You can use emoji on GitHub.com and GitHub:Enterprise's issues, pull requests, commit messages, markdown files e.g. README.md, basically almost anywhere you see a textfield.
It's a pretty effective way of communicating.
See them all in alphanumeric order @ https://github.com/leereilly/emoji