iOS: Appending an uneditable string to the end of a UITextView
- (BOOL)textView:(UITextView *)textView shouldChangeTextInRange:(NSRange)range replacementText:(NSString *)text{
   NSInteger signatureLength=20;
   if(range.location>self.textView.text.length-signatureLength){
       return false;
   }
   else{
       return true;
   }
}

-(void)textViewDidChangeSelection:(UITextView *)textView{
       NSInteger signatureLength=20;
       NSRange newSelection=self.textView.selectedRange;
       if(newSelection.location>self.textView.text.length-signatureLength){
           [self.textView setSelectedRange:NSMakeRange(self.textView.text.length-signatureLength, 0)];
       }
 }