docs(readme): document registration.prefer-jmx variable (#110) (#111)

(cherry picked from commit cbd0d23)

Co-authored-by: Andrew Azores <aazores@redhat.com>

README.md

@@ -45,6 +45,7 @@ and how it advertises itself to a Cryostat server instance. Required properties
 - [ ] `cryostat.agent.webserver.port` [`int`]: the internal port number for the embedded webserver to bind to. Default `9977`.
 - [ ] `cryostat.agent.app.name` [`String`]: a human-friendly name for this application. Default `cryostat-agent`.
 - [ ] `cryostat.agent.app.jmx.port` [`int`]: the JMX RMI port that the application is listening on. The default is to attempt to determine this from the `com.sun.management.jmxremote.port` system property.
+- [ ] `cryostat.agent.registration.prefer-jmx` [`boolean`]: control whether the Agent prefers to publish itself as reachable via JMX or HTTP. If JMX is not enabled on the application JVM then the Agent will always publish itself using HTTP. Default `false`.
 - [ ] `cryostat.agent.registration.retry-ms` [`long`]: the duration in milliseconds between attempts to register with the Cryostat server. Default `5000`.
 - [ ] `cryostat.agent.exit.signals` [`[String]`]: a comma-separated list of signals that the agent should handle. When any of these signals is caught the agent initiates an orderly shutdown, deregistering from the Cryostat server and potentially uploading the latest harvested JFR data. Default `INT,TERM`.
 - [ ] `cryostat.agent.exit.deregistration.timeout-ms` [`long`]: the duration in milliseconds to wait for a response from the Cryostat server when attempting to deregister at shutdown time . Default `3s`.