Update docs for using layer options (#186)
Update docs for using layer options (#186)

`layer:options` was split out from `layer:basic` and a minor change was
made to the API.

See the following for more info:

  * https://discourse.jujucharms.com/t/small-layer-options-api-change/62
  * https://github.com/juju-solutions/layer-options
juju-solutions/charms.reactive,juju-solutions/charms.reactive