GitHub supports Mermaid diagrams and renders them directly in Markdown files.
They aren't rendered in the preview tab, so it's best to design them else-where. With mermaid.live, for instance.
Here's a basic example of the syntax and the resulting diagram:
```mermaid
graph TD;
A[Lunch?]--> B{Is >11:00};
B-- Yes -->C[Nom nom];
B-- No -->D[Wait];
C --> E[Happy!]
D-->A;
```
graph TD;
A[Lunch?]--> B{Is >11:00};
B-- Yes -->C[Nom nom];
B-- No -->D[Wait];
C --> E[Happy!]
D-->A;