[Imports] Proposed new section in import guidelines

[Bryce Nesbitt]

This is a proposed additional topic for "Step 3"
http://wiki.openstreetmap.org/wiki/Import/Guidelines

The goal is to ask respondents to document something they probably are
in a good position to find out::
>>>>How frequently has the source data been updated and by whom?<<<<


And answer one question, meant to get the respondent thinking about maintenance:
>>>>If a future mapper wished to compare an updated dataset, with that previously imported into OSM, how would the two data sets be compared?<<<<