Clarify terms tag / element / attribute in the `README.md`

[sschuberth]

I'm sorry for such a basic question, but I'm getting a bit confused with the terms used in the README.md. I believe "tag" (like <tag></tag>) and "attribute" (like <tag attribute=value></tag>) are quite clear to me, but what is an "element" then? It mostly seems to be an alias for tag, but sometimes also refer to an attribute. Can this be clarified with an initial definition of terms, maybe?

[pdvrieze]

I'll review the text for clarity/consistency. Element means "Element" as used by the library, which more or less corresponds to a property in the Kotlin type.

[sschuberth]

Thanks, any clarification would be appreciated!