Add nl6720 to the list of maintainers

README.rst:
Add nl6720 to the list of maintainers eligible to craft releases using the PGP key with the ID
`BB8E6F1B81CF0BB301D74D1CBF425A01E68B38EF`.
7 jobs for nl6720_developer in 41 minutes and 31 seconds (queued for 9 seconds)