chore: add doc on pixel perfection (#16539)

doc/README.md

@@ -24,6 +24,7 @@
 
 [Patching](patching.md)
 
+[Creating a pixel perfect UI](pixel-perfection.md)
 
 ## Testing
 

doc/pixel-perfection.md

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+# Pixel Perfection
+The Status Mobile team aims to align the design implementation perfectly according to a given screens respective Figma design spec.
+
+Currently the Figma Designs match up with an "iPhone 11 Pro" screen size. 
+
+To test your implementation is correct you can take the following steps
+
+- Open a new Figma file so that you have write privileges. 
+- Copy in the component or screen from the Figma file.
+
+- Using the "iPhone 11 Pro" simulator (it is the default now) you take a screenshot of your component and paste it into your Figma file.
+
+- To get the right size, set your screenshot to width 375px
+
+![](images/pixel-perfection/layer-width.png)
+
+- Lower the opacity so you can see it sitting on top of the Figma mock ups.