Update README.md

add YCRC intro link

Author: j-demayo

README.md

@@ -521,3 +521,7 @@ In this class, we learned:
 * Essential command line tools for reading and editing files.
 * Fundamentals of the permission system.
 * The basics of automating tasks with shell scripting.
+
+  ## Further Learning
+  Yale Center for Research Computing's Practical Introduction to Linux
+  - https://research.computing.yale.edu/training/general-workshops/practical-introduction-linux