Update README.md

Author: PhilJay

README.md

@@ -40,7 +40,7 @@ If you are having questions or problems, feel free to contact me. Since I would
 
  - Open questions on [**stackoverflow**](https://stackoverflow.com/search?q=mpandroidchart) with the `mpandroidchart` tag
  - Search [**known issues**](https://github.com/PhilJay/MPAndroidChart/issues) for your problem (open and closed)
- - Create new issues (if your issue does not exist yet)
+ - Create new issues (if your issue does not exist yet --> please **search known issues before**)
 
 You can let me know via e-mail that you have opened a stackoverflow question so that I might get to answering it more quickly. Thank you.