Add link to pint custom units page to docs

docs/metadata.rst

@@ -79,7 +79,7 @@ Column units are configured in the metadata like so::
     }
 
 Units are interpreted using Pint_, and you can see the full list of available units in
-Pint's `unit registry`_. You can also add custom units to the metadata, which will be
+Pint's `unit registry`_. You can also add `custom units`_ to the metadata, which will be
 registered with Pint::
     
     {
@@ -90,6 +90,7 @@ registered with Pint::
 
 .. _Pint: https://pint.readthedocs.io/
 .. _unit registry: https://github.com/hgrecco/pint/blob/master/pint/default_en.txt
+.. _custom units: http://pint.readthedocs.io/en/latest/defining.html
 
 Setting which columns can be used for sorting
 ---------------------------------------------