Add a paragraph to explain how it works on onedataset page

content/onedataset.html

@@ -167,6 +167,10 @@ draft: false
           update_file(e.target.value);
         }
     </script>
+    <p>
+      In this page you can calculate \(E_\text{adia}\) \(E_\text{0-0}\) from \(E_\text{abs}\) \(E_\text{fluo}\) and \(E_\text{\Delta ZPE}\)
+      For this select a method for absorption, fluorescence or \(\Delta \text{ZPE}\)
+    </p> 
     <div>
     <label>Select a molecule<br/>
       <select id="select_mol" onchange="select_mol_onchange(event)">