Added warning in documentation in accordance with issue 822 of the oxyplot repository.

Author: Isolocis

export/export-png.rst

@@ -27,6 +27,8 @@ Write to a file
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 ---------------------
 
+*Warning: Calling the ExportToBitmap method often in a very short period may result in high memory usage.*
+
 .. code:: csharp
 
     var pngExporter = new PngExporter { Width = 600, Height = 400, Background = OxyColors.White };