Add Wakatime Badge

Summary: Fixes T318. Add it instead of replacing it, as originally planned.

Test Plan: Tested via GitHub Gist that it displays the correct badge in correct location.

Reviewers: ^ACL_sudoers, Code Meister, revi

Reviewed By: ^ACL_sudoers, revi

Maniphest Tasks: T318

Differential Revision: https://bugs.revi.xyz/D14