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