Summary should be level 3 header.
Join two KGTK edge files or two KGTK node files.
-## Summary
+### Summary
* Join keys are extracted from one or both input files and stored in memory, then the data files are processed in a second pass. Performance will be poor, and execution may fail, if the files are very large.
* stdin will not work as an input file if join keys are needed from it.
