top.1: Add HISTORY

top was added in 511d9c656578 for FreeBSD 2.2.2.

MFC after: 3 days
Reviewed by: carlavilla, imp, joerg
Approved by: carlavilla (mentor)
Differential Revision: https://reviews.freebsd.org/D49591

(cherry picked from commit a9f5dcf82941e7e211490432b93bd124e7c0a57d)