Wiki - add info about the need of proper ContentType header for POST APIs
"In POST requests use Content-Type: application/json header." - this info is missing in wiki page.
see https://forums.openlp.org/discussion/5925/http-api-v2-question
"In POST requests use Content-Type: application/json header." - this info is missing in wiki page.
see https://forums.openlp.org/discussion/5925/http-api-v2-question