Update README.md

Author: zx8754

README.md

@@ -36,6 +36,9 @@
  - `[Group by distinct count](https://stackoverflow.com/questions/12840294)`
  - `[Group by top_n](http://stackoverflow.com/questions/27766054)`
 
+### %functions%
+ - `[What do the %op% operators in mean in R? for example “%in%”](https://stackoverflow.com/q/12730629)`
+ 
 ### Reshape:  
  - `[Reshape data from long to wide format](http://stackoverflow.com/questions/5890584)`   
  - `[Reshape data from wide to long format](http://stackoverflow.com/questions/2185252)`