new committer (src): Pierre Pronchery (khorben)
per the committer's guide for new committers:
- add my pgp key
- add myself to shared/authors.adoc
- remove myself from shared/contrib-additional.adoc
- add myself to shared/contrib-committers.adoc
- add a news entry
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D50250