Add URL Scheme documentation

Technotes/Themes.md

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+# Themes
+
+## Add Themes Directly to NetNewsWire
+Theme developers: on iOS and macOS, themes can be opened directly in NetNewsWire using the below URL scheme:
+
+`netnewswire://theme/add?url={url}`
+
+When using this URL scheme the theme being shared must be zipped.
+
+Parameters:
+- `url`: (mandatory, URL-encoded): The theme's location.