Update README.md

Author: SteveWilkes

README.md

@@ -104,4 +104,11 @@ To define a custom factory for translating `ConstantExpression` values, use:
 string readable = myExpression
     .ToReadableString(c => c.TranslateConstantsUsing(
         (constantType, constantValue) => GetConstantValue(constantType, constantValue)));
-```
+```
+
+To specify a custom string for code indenting, use:
+
+```csharp
+string readable = myExpression
+    .ToReadableString(c => c.IndentUsing("\t"));
+```