expand README to include information about external reference support

[chris-armstrong]

As per #573 (comment), add some documentation about external reference support

[codecov]

Codecov Report

Merging #574 (5c95ff3) into master (224e8b0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #574   +/-   ##
=======================================
  Coverage   90.85%   90.85%           
=======================================
  Files         107      107           
  Lines        1323     1323           
  Branches      235      235           
=======================================
  Hits         1202     1202           
  Misses        121      121           

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