Add warning to uppercase option

Author: nene

README.md

@@ -54,7 +54,7 @@ You can also pass in configuration options:
 format('SELECT * FROM tbl', {
   language: 'spark', // Defaults to "sql" (see the above list of supported dialects)
   indent: '    ', // Defaults to two spaces
-  uppercase: bool, // Defaults to false
+  uppercase: bool, // Defaults to false (not safe to use when SQL dialect has case-sensitive identifiers)
   linesBetweenQueries: 2, // Defaults to 1
 });
 ```