Spell out review-within-two-weeks rule for Code Owners

[tautschnig]

Pull requests that do not receive timely feedback block progress and cause
unnecessary overhead as repeated rebasing is necessary and increasingly costly.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: fe06991).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90297063

[martin-cs]

I don't suppose anyone feels like writing some kind of kick script to enforce this do they?