added links to core dev team members

Author: chrissimpkins

CONTRIBUTORS.md

@@ -4,9 +4,9 @@
 
 ### Core Development Team
 
-- Chris Simpkins (@chrissimpkins) - project lead
-- David van Gemeren (@burodepeper) - typeface usability
-- JM (@jublo) - typeface design, web development
+- Chris Simpkins ([@chrissimpkins](https://github.com/chrissimpkins)) - project lead
+- David van Gemeren ([@burodepeper](https://github.com/burodepeper)) - typeface usability
+- JM ([@jublo](https://github.com/jublo)) - typeface design, web development
 
 ### Hinting / Instruction Sets
 - Werner Lemberg