Mention tagName option into README.md

[ansmonjol]

As mentioned in this issue, this seems to render the markdown without the ember-view div wrapper.

Didn't tried yet, just saw that opened issue that may be worth mentioning in the readme.

Maybe @sheriffderek you can confirm this worked for you ?

Of course, feel free to suggest update for the wording or code example provided here if you feel it can be better 😃

[mansona]

This is actually not possible any more 😞 I think we could do a PR that actually removes the surrounding div that we added when moving to glimmer and release it as a major if people are still interested