Add docs chapter on default units and physical constants (#729)

* add Pre-defined units and constants  chapter

* rename "Dimensionful" logarithmic units

Author: Eben60

docs/make.jl

@@ -21,6 +21,7 @@ makedocs(
         "Interoperability with `Dates`" => "dates.md"
         "Extending Unitful" => "extending.md"
         "Troubleshooting" => "trouble.md"
+        "Pre-defined units and constants" => "defaultunits.md"
         "License" => "LICENSE.md"
     ]
 )