Clarify how start and end dates for charts are derived in the documentation...

The documentation doesn't make it very clear how start and end dates for charts are determined. It would be helpful to update the documentation with an explanation and/or diagram. Here's a summary based on recent conversations with JC that I hope is a good starting point.

The start and end dates for GreenHopper charts are determined roughly as follows:

1. Before preparing the chart, issues are filtered as follows:

• Issues that aren't in a GreenHopper-enabled are excluded
• Issues that don't have at least one unreleased fix version selected are excluded
  2. The list of versions is created based on the fix version associated with issues included in the filtered list.
  3. The earliest start date is selected from the list of versions.
  4. The latest end date is selected from the list of versions.

The end date for each version is determined by looking at (in order):

1. The end date configured in the planning board when viewing the version
2. The release date of the version
3. Today's date