If you want your mod to be available in the 0 A.D. mod downloader, you need to follow these steps.
1) Prepare your mod as explained in the Modding Guide. You should end up with a .zip file. If you created a .pyromod file, copy it and rename the copy to modname.zip so it can be uploaded on mod.io.
2) Sign up or login on mod.io. Go to Mods > Add Mod, and specify 0 A.D. in the field Game. Add all the information you want to make available for visitors. Complete the submission steps until your mod is shown as waiting for moderator approval.
3) When you are ready, send me a PM here on the forums, or an email. I will get the mod from mod.io, test it and take a look at its code. When I have checked the mod is not dangerous, I will sign the file. I will answer the PM/email with the signature. I can also report bugs in the mod, and mistakes you may have made when uploading the file on mod.io.
I try to consistently dedicate Wednesday afternoons to mod signing.
Important Notice:
Wildfire Games signs mods in order to mitigate the attacks that can be conducted against file repositories and the clients that download from them. Signing a file does not imply that the mod is official, nor that Wildfire Games participated in its creation in any way. It does not mean that it is exempt of bugs or oversights either. The only guarantee signing brings is that the file you are downloading is exactly the one that the signatory tested.
4) I will put the signature in the metadata of the file on mod.io, then make the mod live. Right after this is done, your mod will appear in the mod downloader!
Happy modding