Hidden markdown text on GitHub

Question

Is there anything in markdown syntax specifically on GitHub to support hidden text?

I just want to put some to-do notes in README.md for myself, not to be visible.

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EXTRAS

As a tribute to the great answer by Tamas, and what's also asked a lot of times, below is an example of how to write foldable sections within MD files:

<details> <summary><b>My section header in bold</b></summary>  Any folded content here. It requires an empty line just above it.  </details> 

Answer 1

Just use standard HTML comments:

<!-- This is commented out. -->