XD-1560 Improve README.md
XD-1560 Improve README.md

* Add `--contentType "application/json"` when doing a `http post` to ensure payload is of type JSON

viveksd87/spring-xd-samples,ghillert/spring-xd-samples,huunhancit/spring-xd-samples,spring-projects/spring-xd-samples,trisberg/spring-xd-samples,rajkumargithub/spring-xd-samples,spring-projects/spring-xd-samples,trisberg/spring-xd-samples,viveksd87/spring-xd-samples,ghillert/spring-xd-samples,felipeg48/spring-xd-samples,constantlearner/spring-xd,huunhancit/spring-xd-samples,huunhancit/spring-xd-samples,rajkumargithub/spring-xd-samples,huunhancit/spring-xd-samples,felipeg48/spring-xd-samples,spring-projects/spring-xd-samples,viveksd87/spring-xd-samples,trisberg/spring-xd-samples,trisberg/spring-xd-samples,constantlearner/spring-xd,constantlearner/spring-xd,morfeo8marc/spring-xd-samples,trisberg/spring-xd-samples,morfeo8marc/spring-xd-samples,spring-projects/spring-xd-samples,felipeg48/spring-xd-samples,rajkumargithub/spring-xd-samples,morfeo8marc/spring-xd-samples,morfeo8marc/spring-xd-samples,spring-projects/spring-xd-samples,trisberg/spring-xd-samples,morfeo8marc/spring-xd-samples,viveksd87/spring-xd-samples,viveksd87/spring-xd-samples,felipeg48/spring-xd-samples,ghillert/spring-xd-samples,morfeo8marc/spring-xd-samples,ghillert/spring-xd-samples,rajkumargithub/spring-xd-samples,ghillert/spring-xd-samples,viveksd87/spring-xd-samples,constantlearner/spring-xd,ghillert/spring-xd-samples,rajkumargithub/spring-xd-samples,spring-projects/spring-xd-samples,huunhancit/spring-xd-samples,constantlearner/spring-xd,huunhancit/spring-xd-samples,constantlearner/spring-xd,felipeg48/spring-xd-samples,rajkumargithub/spring-xd-samples,felipeg48/spring-xd-samples