added README and screenshot

README.md

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+# Sketch Palettes
+
+This plugin for saving, loading, and clearing color palettes in the Document Colors Section of the color picker.
+
+<img src="palettes.png" style="width: 100%; max-width:800px;" />
+
+# Installation
+
+Move the Sketch Palettes plugin into your Plugins folder. You can get access to that folder by opening the Plugins menu, and choosing "Reveal Plugins Folder..."
+
+
+# Usage
+In the Sketch Palettes plugin menu are three actions:
+
+#### Saving Palettes
+
+Select "Save Palette..." from the plugin menu. The plugin will grab the colors that are currently in the Document Colors section of the color picker in the current document. Then it will save a .sketchpalette file to whatever location you choose.
+
+_Note: If you have the App Store version of Sketch, because of Apple's additonal security features, a dialog will pop up asking you to authorize access the folder you want to save to. You will only ever have to do this once. The version of Sketch downloaded directly from the website does not require this._
+
+#### Loading Palettes
+
+Select "Load Palette..." from the plugin menu. Open a .sketchpalette file containing the colors you want to load into the Document Colors section of the color picker. This will replace whatever colors that are currently there.
+
+#### Removing Palettes
+
+Selecting "Clear Palette" will remove all colors from the Document Colors section of the color picker.
+
+# Info & Feedback
+
+If you have any questions, find a bug, or have ideas for ways to improve the plugin, ping me on twitter: <a href="https://twitter.com/AndrewFiorillo" target="_blank">@andrewfiorillo</a>
+
+Also, big thanks to <a href="https://twitter.com/wbobeirne" target="_blank">William O'Beirne</a> for helping me get through some of the trickier parts, like making sense of the <a href="http://stevenygard.com/projects/class-dump/" target="_blank">class-dump</a> of Sketch, to figure out how to use undocumented Sketch functionality.