Add a new packager key for serebit

Add a new packager key

Details

• Username: @serebit
• PGP key ID: E87E5B39F04A5D889D8C0147F6D84143496F6680

Related issue: #243 (closed)

Checks

• All steps in the accompanying ticket are fulfilled.

Main key holders

• The public key has been validated according to the best practices