Added theme issue template

ISSUE_TEMPLATE.md

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+Welcome!
+
+Want to add a Kirby theme? How awesome!
+
+1. Make sure the theme is compatible with Kirby 2.
+2. Make sure it's not already in the repository by searching for the url.
+
+#### Format (Optional)
+
+Just want to add a link? Great! Do that!
+
+Want do add more formatting? Here is a formatting suggestion. Replace the brackets with content.
+
+**Site:** [theme website]
+**Github:** [theme repository on github]
+**Demo:** [theme demo]
+
+[screenshot image]
+
+[quote or short information]