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There's no centralized sharing hub, but you can upload it to wherever you prefer and post a link right here in this forum. 

I recommend posting a screenshot too and perhaps a short description, so that people know what they can expect when downloading it.

(You can use the small .PNG file that is in the level folder as screenshot)

 I recommend first creating a .ZIP file of your level folder.

To those reading this who doesn't use Github, the easiest place to upload your level is probably Google Drive. You can just drag and drop your zip file to your google drive account, right-click the file and select "Get shareable link" and post that link here. (You can also use another file sharing service such as Dropbox or Jumpshare, whatever you prefer).