README.md: Add note about naming scheme.

README.md

@@ -9,9 +9,13 @@ Download
 See: https://w3.math.uzh.ch/qfq/
 
  * release - Stable
- * nightly - Development version
- * snapshot - Current, development version 
+ * nightly - Development version(1)
+ * snapshot - Current, development version(1)
 
+(1) To install the extension, the name of uploaded ZIP file has to follow some rule:
+
+ * <ext>.zip or
+ * <ext>_<major>.<minor>[.<micro>][_<something].zip
 
 Documentation
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