Add content to TEACHING.md

TEACHING.md

-# Teaching tips
+# Teaching
 
-## White-boards
+## Teaching
+
+There are three important aspects to teaching:
+
+1. Teach general principles (concepts) of how things work
+2. Teach methods of how to practically do things
+3. Answer student's questions
+
+## Teaching methods
+
+### White-boards
 
 - Try to frequently use white-board, because:
 	- It makes teaching more interactive.
@@ -10,7 +20,7 @@
 	- One for tidy concept maps.
 	- One for messy notes.
 
-## Stand
+### Stand
 
 - Try not to sit, because:
 	- Teaching will be more dynamic.