Request: Improve documentation for `openWhenHidden` property

The default for `openWhenHidden` is false. This means that the SSE will abort on the client side if the document is hidden, however the default SSE behaviour is that the SSE will continue even if the document is hidden.

Since this is different then normal SSE, I think there should be something in the readme that says something along the lines of "To continue SSE streaming when browser is hidden, set `openWhenHidden` flag to true". I'm happy to do so if this is approved.

Thank you!