Add committer entries in doc for Lutz Donnerhacke <donner>

[donner]

Summary

Steps 1-4 from the Committer's guide:

• Add entries for Lutz Donnerhacke <donner>
• Add GPG key

[donner]

I do not have access to freefall at the moment, but from the current developer list, I'd assume, the name "donner" is free to be assigned.
The name is also my IRC-nick, so I'd like to keep them consistent.

[donner]

I'd suggest to use the following commit message

Add committer entries for Lutz Donnerhacke <donner>

Steps 1-4 from the Committer's guide:
  - Add entries for Lutz Donnerhacke <donner>
  - Add GPG key

Reviewed by:            kp, philip
Approved by:            kp (mentor), philip (mentor)
Differential Revision:  https://reviews.freebsd.org/D27967

[donner]

• Change OpenPGP key
• Update the day of the commit bit

[kp]

LGTM

[philip]

Looks good to me.

Welcome to the party! Glad to have you on board.

[donner]

Rebase to 31f481ca1c384b8c