Add example of preference key construction

[mdavidsaver]

Adds an example of how package and preference names are concatenated in a settings.ini file.

[kasemir]

Yes, good to give an example there.
The build process also creates a settings_template.ini file that contains all the descriptions and the default setting as an example:

##########################################
# Package org.csstudio.trends.databrowser3
##########################################
...
# If there is an error in retrieving archived data,
...
# org.csstudio.trends.databrowser3/drop_failed_archives=true