Add Readme and screenshot

README.md

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+# Defold Polygon Editor
+
+Since [it's possible to use convex polygon collision shapes with Defold](https://forum.defold.com/t/does-defold-support-only-three-shapes-for-collision-solved/1985), but the Editor doesn't yet support creating and editing them, I wrote a little program(with Defold) to fill that gap. So you no longer have to make a text file and write in vertex coordinates, in counter-clockwise order, by hand!
+
+It's very, very simple and unpolished. Here is a short list of features:
+- Load an image as a reference for your polygon (unscaled and centered at origin).
+- Open and save polygon files.
+    - Uses [Def-Diags](https://github.com/andsve/def-diags/) for native file dialogs on Windows and Mac, and tries to use Zenity on Linux.
+    - Can still save polygons in the application folder if none of those work.
+- Highlights convex (and therefore invalid) vertices in red.
+- Hold shift to snap vertices to 5-pixel increments.
+
+[alt text](https://github.com/rgrams/defold_polygon_editor/screenshot.png "Editor screenshot")