Added Custom folder and readme.md fixes DNNCommunity#598

Dnn.CommunityForums/themes/community-default/custom/README.md

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+# A way to overule Forum Theme Styles 
+
+**New DNN Community Forums Module v 08.00.00**
+
+To overrule a Forum Themes Styling, you can create a folder named "custom" and place a file "theme.css" in it.
+The Forums module will load this custom/theme.css Styleheet if it exists. 
+This is a way to overrule some default Styling of a Forum Theme. 
+
+###### Warning on the Forum Module Default Themes
+
+> When you upgrade the Module:
+> - The "custom/theme.css" file will not be overwritten.
+> - All other files of the default Forum Themes will be overwritten though. 
+
+So when you need to edit more than just "custom/theme.css"; 
+- Make a copy of the Theme folder first and make your changes in there. 
+
+**This will avoid your changes getting overwritten on a Module upgrade.**
+
+See also: https://github.com/DNNCommunity/Dnn.CommunityForums/wiki/Creating-Themes
+