Add documentation for event writing

[matthewfeickert]

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My only comment to you @APN-Pucky is that the functionality should be visible as an example notebook for sure, probably in a dedicated new notebook (otherwise many will not realise writing is now available). Likewise I would add a little paragraph/sentence in the README.

Originally posted by @eduardo-rodrigues in https://github.com/scikit-hep/pylhe/issues/233#issuecomment-2309825784