Added ChooseTheme plugin to the repository

plugins/ChooseTheme/README.md

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+### Choose theme
+A simple plugin for [GNU social software](http://gnu.io/social/). 
+The plugin enables the user to select their own theme, independently on site setting and other users.
+Consider this plugin as experimental.
+
+#### Enable plugin
+- Include this code in your GNU social instance. 
+- Edit your `config.php` to include `addPlugin("ChooseTheme");`
+(NB: Your plugin directory must then be named ChooseTheme or create a symbolic link to your cloned directory)
+
+#### How-to
+- Choose settings from the GNU social menu. Choose 'Theme' on left menu.
+- Select a theme and press 'Save'.
+
+