APPNG-2115: Minor updates to README.adoc
APPNG-2115: Minor updates to README.adoc

- Removed the VersionEye badge, due to https://blog.versioneye.com/2017/10/19/versioneye-sunset-process/
- Added Camunda BPMN support to the feature list
- Add link to contributing guidelines. See also APPNG-2005
- Add link to appNG Application Repository - Stable

appNG/appng,appNG/appng,appNG/appng