Add site info

de.hftstuttgart.cityunits.p2site/site.xml

 <?xml version="1.0" encoding="UTF-8"?>
 <site>
+   <description name="City Units" url="https://transfer.hft-stuttgart.de/pages/neqmodplus/de.hft-stuttgart.cityunits/p2repo">
+      Eclipse P2 repository with Ecore data types Quantity for units based on Indriya reference implementation of Units of Measurement Java specification (JSR 385) and some special units for urban simulation.
+   </description>
    <feature url="features/de.hftstuttgart.cityunits.feature_1.2.0.jar" id="de.hftstuttgart.cityunits.feature" version="1.2.0">
       <category name="de.hftstuttgart.cityunits"/>
    </feature>