docs: clarify the use of Importer properties in post processing steps

Signed-off-by: David Greaves <david@dgreaves.com>
Merge pull request #221 from lbt/master

docs: clarify the use of Importer properties in post processing steps
Merge pull request #223 from VirusFree/master

[FBX ] Use AiNode's metadata to keep objects UserDefinedProperties