add missing dependent key in README

[marcoow]

The headers CP in the adapter example relies on both this.session.data.authenticated.access_token and this.session.isAuthenticated but only specifies session.data.authenticated.access_token as a dependent key which triggers a linter error:

Use of undeclared dependencies in computed property: session.isAuthenticated ember/require-computed-property-dependencies

This adds the missing key to the README.

addresses (part of) #2289