Add most recent plugin development documentation.

This document has not been updated to include the most recent
changes to the plugin spec, which is being migrated to the wiki,
but will serve as a starting point for those interested in plugin
development
Merge pull request #248 from CoastalResilienceNetwork/topic/add-docs

Add most recent plugin development documentation.
Feature compare has default display for nulls

Feature compare results table showing 'gaps' due to null values.  Places
default text to have all cells filled in.
Merge pull request #249 from CoastalResilienceNetwork/fix/null-comparer

Feature compare has default display for nulls
Feature Compare attr order defined by config

Rather than allowing the order from the mxd to determine the display
order
user the ordering of the fields specs in the compare config
Set plugin window below app modals

Both save 'n share and export had a lower z-index, resulting in the
plugin window being above the modal overlay.  Plugin window is sitll
above all the other map UI elements.
Merge pull request #250 from mmcfarland/fix/fc-field-order

Feature Compare field order