add comment for cJSON_Minify function

cJSON.h

@@ -250,7 +250,9 @@ The item->next and ->prev pointers are always zero on return from Duplicate. */
  * case_sensitive determines if object keys are treated case sensitive (1) or case insensitive (0) */
 CJSON_PUBLIC(cJSON_bool) cJSON_Compare(const cJSON * const a, const cJSON * const b, const cJSON_bool case_sensitive);
 
-
+/* Minify a strings, remove blank characters(such as ' ', '\t', '\r', '\n') from strings */
+/* The input pointer json cannot point to a read-only address area, such as a string constant, 
+ * but should point to a readable and writable adress area. */
 CJSON_PUBLIC(void) cJSON_Minify(char *json);
 
 /* Helper functions for creating and adding items to an object at the same time.