readme: document content-type requirement for metadata
readme: document content-type requirement for metadata

Vagrant requires that metadata be served with `Content-Type: application/json`. Note this in the README to hopefully save our users some frustration.

Fixes #3.
ConduceInc/vagrant-gsauth,WhoopInc/vagrant-s3auth,ConduceInc/vagrant-gsauth,WhoopInc/vagrant-s3auth