When adding a revision of an extension in the Extensions gallery, there is the option to do this via an SVN URL. To do so, follow the steps detailed below.

1. Go to the home page of your extension
2. Click the 'SVN Configuration' button
3. Fill in the field 'Repository URL' and leave the other fields as they are
4. Click the 'Submit' button

Now you are ready to add revisions via SVN. You can do so by

1. Clicking the 'Add a revision' button
2. At the second option (called 'File') click the radio button 'Use SVN file creation'

That's it. You're done!