Update contributors

I sometimes close pull requests yet use parts of the suggested code. In the
end it's a shame that original submitters are not given credit, so this
commit is just a sincere "thank you" to:
- @dagee3 for YOURLS#2362
- @MarcProe for YOURLS#2445
- @hangxingliu for YOURLS#2545
- @Art4 for YOURLS#2581
- @pcolmer / @philip-linaro for YOURLS#2608
- @muellerlukas for YOURLS#2554

Co-authored-by: Darrell Agee <[email protected]>
Co-authored-by: Marcus <[email protected]>
Co-authored-by: Liu Yue <[email protected]>
Co-authored-by: Artur Weigandt <[email protected]>
Co-authored-by: Philip Colmer <[email protected]>
Co-authored-by: Lukas Müller <[email protected]>

(hoping they'll show in the contributors list)