Improvements to externs readme
Merge pull request #2139 from probins/patch-2

Improvements to externs readme.
Clarify compiler exports in tasks readme
Export method to get resolution for extent and size

While this is straightforward to calculate, this method adds convenience.
Merge pull request #2141 from probins/patch-5

Clarify compiler exports in tasks readme.
Merge pull request #2142 from tschaub/res-for-extent

Export method to get resolution for extent and size.
Apidoc readme: clarify exportable methods