Add instructions for custom functions configuration in README

Author: xterao

README.md

@@ -97,4 +97,11 @@ Some functions of "Doma Tools" can be customized from the settings screen.
 - Customize action shortcut keys
 - Toggle the SQL formatting feature on or off
 - Specify the class names that define custom functions.
- ![setting.png](images/setting.png)
+
+**If you want to use custom functions defined in your own ExpressionFunctions implementation class, 
+place a `doma.compile.config` file directly under the resources directory and describe the `doma.expr.functions` entry.**
+  
+ex) doma.compile.config
+```properties
+doma.expr.functions=example.expression.MyExpressionFunctions
+```