Try:
<details>
<summary>Your header here! (Click to expand)</summary>
Your content here...
> markup like blockquote's should even work on github!
more content here...
</details>
You can try this sort of thing here:
<details>
<summary>Your header here! (Click to expand)</summary>
Your content here...</br>
(markup only where supported)</br>
more content here...</br>
</details>
This works for me with Chrome, but may not work yet for other browsers.
There are some related posts at github.