doc: Add a note on PO editors

doc: Add a note on PO editors

While PO files can be edited in any text editor, specialized tools for
translation editing can be quite helpful with automating tasks etc. Add
a small note about PO editors to encourage new translators to research
which tool works best for them.

Reviewed-by: Bruce Momjian <bruce@momjian.us>
Discussion: https://postgr.es/m/163490116698.684.10398197970578456928@wrigleys.postgresql.org