Adding a notice

Author: pH-7

README

@@ -3,6 +3,8 @@
 Simple calculator written in Java with Eclipse.
 This calculator is simple with an easy code to help novices learn how to operate a calculator.
 
+If you use the executable file "calculator.jar" and that it does not run, you can type "java -jar /your_calculator_directory/calculator.jar" in your terminal.
+
 ========== AUTHOR ==========
 
 Pierre-Henry SORIA