Update README.md

[kvitor123]

Added to music theory pages; trying to find out how to properly do a pull request

[levinericzimmermann]

Thanks @kvitor123.

Unfortunately there is a space too much between the square bracket and the round bracket, it should be [Music Theory for Musicians](https://www.music-theory-for-musicians.com/) instead of [Music Theory for Musicians] (https://www.music-theory-for-musicians.com/). With this space, the markdown code isn't correctly rendered as a link.

Once this is corrected, this looks fine to me.

[kvitor123]

@levinericzimmermann Fixed the spacing. Can the code be rendered as a link now? Let me know if it doesn't work still.

[levinericzimmermann]

Thanks @kvitor123, it works now! Maybe just please squash your commits into one, as this is semantically only 1 change ("Add Music Theory for Musicians").