Add CONTRIBUTING.md file.

[FIXED JENKINS-28111] JNLP slave JVM Options are inconsistently applied

[FIXED JENKINS-28115] Division by zero in Executor.getProgress()

```
WARNING: Caught exception evaluating: executor.progress in /ajaxExecutors. Reason: java.lang.reflect.InvocationTargetException
java.lang.reflect.InvocationTargetException
...
	at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.ArithmeticException: / by zero
	at hudson.model.Executor.getProgress(Executor.java:649)
	... 154 more
```

https://github.com/blog/1184-contributing-guidelines

Now that packaging is in https://github.com/jenkinsci/packaging and give...

[JENKINS-28062] Channel listener onClose propagated exceptions

[JENKINS-28058] Don't send a reference to Computer.class over remoting channels

Match the overriden method's actual varargs signature.

Change AsynchPeople column from last active to last commit activity.

Remove unnecessary null check.

[JENKINS-28013,JENKINS-28012] - Revert changes for JENKINS-10629

[FIXED JENKINS-28062] A Launcher.afterDisconnect() method that propagates an exception is a sign of a bad Launcher not a problem closing the channel.

- Logging added to help diagnose was actually causing Jenkins.class to be initialized on the remoting side

This reverts commit b6c7b83e.

This reverts commit 4ddeeb84.

Add @Deprecated annotations to things documented as @deprecated.

Advertise X-JNLP-Host-Name correctly

This allows TCP slave agent port to be reverse proxied.

Now that packaging is in https://github.com/jenkinsci/packaging and given that the ips packaging is stale, we should just drop this

This prevents a long running CLI process (such as CloudBees OPE
proccesses) to hang forever when the main thread fails.

Whereas if the process terminates cleanly, the wrapper (such as systemd)
can take corrective actions.

Signed-off-by: NKamran Mackey <kamranm1200@gmail.com>

Signed-off-by: NKamran Mackey <kamranm1200@gmail.com>

[JENKINS-27918] Downgrade warning from Stapler