Clarify Purpose of Extension Definition

[jyutzler]

While looking at opengeospatial/geopackage-tiled-gridded-coverage#20 we determined that there was no point in making this definition machine-readable. Rather than remove the column entirely, we decided to make this column human-readable.

[jyutzler]

After looking at this more closely, I believe the best use of the column is to put a URL defining the extension, e.g., http://www.geopackage.org/spec/#extension_metadata

[jyutzler]

Based on opengeospatial/geopackage-tiled-gridded-coverage#20 (comment) we don't have consensus yet.

Let's talk this through on Wednesday then. I will prepare a pull request but I won't merge it yet.

[jyutzler]

The pull request (strawman) says "permalink, URI, or reference..."