Update Readme

[LuciooF]

Link to ZD API is broken.

"The zendesk api documentation is available at http://developer.zendesk.com/documentation/rest_api/introduction.html Querying and searching is limited by the searchable fields on the zendesk api"

"http://developer.zendesk.com/documentation/rest_api/introduction.html" does not work.
I believe correct link should be "https://developer.zendesk.com/rest_api/docs/api-clients/introduction"

[mikerogers123]

Hi @LuciooF, our team has recently taken some level of ownership of this repository. I have added a TODO card for this. We plan on prioritising the work as part of our workflow so we should get round to it. Until then, PRs are welcome 👍