If you want to thank your co-creators, or the creators of the workshop items you’ve used, you can add a Text Effect clip to your Overlay Effects track. You can set the text by right clicking on the clip and selecting Edit Clip -> Set Clip Text. To edit other properties of the text effect clip, right click on it and select Show in Element Viewer, and then edit those properties there. Alignment is determined by horizontalAlignment and verticalAlignment, for which 0 means align centered, -1 means align left (or top) and 1 means align right (or bottom). Fonts are chosen from among those listed in platform/resource/boxrocket.res.
To get the list of all workshop items you’ve used in your session, you can select File -> Sharing -> Show Used Workshop Items. This will show you all the workshop items used, grouped by the creator of the items. You can click on any of the links in that dialog to see the steam page for the appropriate items or users. You can also copy and paste that text into a Text Effect clip, to get you started on your credits.