Update kill timeout documentation

[jarischaefer]

The documentation should be updated to explain the increased kill timeout (see PR 510).

From CONTRIBUTING.md

The master branch is supposed to point to the latest stable release, because users read documentation from the master branch.

Should we update the documentation on the master branch despite the fact that the current release does not yet reflect the changes made in master? Perhaps a disclaimer should be added (e.g. "This change is only available on 0.11.1+").

[kingdonb]

Maybe something under the heading:
https://github.com/phusion/baseimage-docker/#shutting-down-your-process

Like, "Prior to 0.11.1, the default value for KILL_PROCESS_TIMEOUT was 5 seconds. In version 0.11.1+ the default process timeout has been adjusted to 30 seconds."

Since it was always a user-configurable setting, I don't think language like "only available" makes sense. (Opening a PR for this)

[kingdonb]

How does that look

[jarischaefer]

Looks good to me