add documentation for adding new themes to the theme gallery.

Author: bplcoder14

ADDING_THEMES.md

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+How to add a new theme to the theme gallery:
+
+1. Take a screenshot of the theme and place it in the /images/gallery/themes folder. Make sure the name of the screenshot is like this: (theme name).png
+
+2. Add the name of the theme to the ```themeList``` array in ```load_themes.js```.