Extended description for eXIf

Hi,

I published the following document as a gist on BitBucket:

EXIF Support In PNG
https://bitbucket.org/snippets/ctruta/aLd8A

Many thanks to Glenn for the initial review.

I would like to publish it further on PNG-Tech, and it would be a good
idea to attach it (or something similar to it) to the upcoming Call
for Vote; but meanwhile, I will appreciate your feedback, either on
this list, or in the gist's comments section.

Sample test images are coming up, probably over this weekend.

Sincerely,
Cosmin

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