[docs] Add usage and minimum nuxt version

[pi0]

It would be nice adding some usage example to main README. Also i think as of additionalExtensions usage, some older nuxt versions would partially work. /cc @kevinmarrec i can't recall exact version

[codebender828]

Thank you @pi0 ! That's a good idea. Let me add some information on the README.md about usage. Not sure I fully understood what you meant about the additionalExtensions usage. Is there something internal that's changing regarding the additionalExtensions API for older versions?

[pi0]

Extending supported extensions added by nuxt/nuxt#5854 via v2.10.0 so it is essentially minimum supported version for this module.

PS: I think now we have docs we can empty main README and link to docs ;)

[codebender828]

Noted. Will update the README and link to docs. Thank you! @pi0 @Atinux