Describe server loading options in server docs

Wanted to document options for overriding the default server loader as per #376. I've already signed the typesafe CLA, but maybe not for sbt-native-packager.
I think it's better to recommend appending to this setting, in case some other plugin wants to add its own (hopefully compatible) lines.

Also, it seems like it might be more consistent to make use of the same IO library throughout the examples, and `sbt.IO` is used in the next example.

An alternative (untested) could be:
```scala
bashScriptExtraDefines ++= IO.readLinesURL(this.getClass.getResource("/setup-env"))
```