Added a few designer notes

DESIGNER_NOTES.md

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+* Expressions are not supported.
+* Text is not supported.
+* If you have an Illustrator layer, convert it to a shape layer or else it will be exported as a 
+bitmap.
+* The smaller the content in a masked or matted layer is, the better the performance will be.
+* Opacity is applied to individual elements not entire layers/groups. This can affect the 
+appearance of overlapping items when the group or layer has opacity.