Create documentation for the ICDO concept decisions

[sratwani]

Per the meeting on 11/7 and following the completion of tasks #67, #146, #176, Rimma recommended that we add documentation for the ICD0 concept decisions.

[ekorchmar]

I have updated OHDSI wiki entry for the structure of the vocabulary itself.

https://www.ohdsi.org/web/wiki/doku.php?id=documentation:vocabulary:icdo3&s[]=icdo3

At this moment link to github from the article will send you to 404. That's because code is resided in a branch and had not been yet merged to master.

Actual code can be found here:

https://github.com/OHDSI/Vocabulary-v5.0/tree/icdo3/ICDO3

Please let me know if you have any questions or if I have missed anything in the document.

[mgurley]

@ekorchmar @dimshitc
Closing #139 based on this comment.