Add some more explanation on how to add projects to TWIM

Something like this at the end would be great:

> # That's all I know
>
> See you next week, and be sure to stop by #twim:matrix.org with your updates!
>
> To add your update to the next edition, drop a message prefixed by a mention to the user `TWIM` in the above room, and it will be picked up. For images, please also mark them with a ⭕️ reaction.

Clarification: This issue is about updating the TWIM-Bot template. Not about updating existing posts

Locate the TWIM-Bot template in the matrix.org repository and review how its closing section is assembled. Update the template with the proposed instructions for submitting updates and images, while leaving existing posts unchanged; verify the template renders correctly.

Written by the indexing model from the issue text.