fix: Add preview reference to the createClient

Author: trodrigues

lib/contentful.js

@@ -16,7 +16,7 @@ import createCdaApi from './create-cda-api'
  * @prop {string} params.space - Space ID
  * @prop {string} params.accessToken - Contentful CDA Access Token
  * @prop {boolean=} params.insecure - Requests will be made over http instead of the default https (default: true)
- * @prop {string=} params.host - API host (default: cda.contentful.com)
+ * @prop {string=} params.host - API host (default: cda.contentful.com). Also usable with preview.contentful.com.
  * @prop {Object=} params.agent - Optional Node.js HTTP agent for proxying (see <a href="https://nodejs.org/api/http.html#http_class_http_agent">Node.js docs</a> and <a href="https://www.npmjs.com/package/https-proxy-agent">https-proxy-agent</a>)
  * @prop {Object=} params.headers - Optional additional headers
  * @returns {CDAClient.ClientAPI}