Document upgrade path for third-party extensions

Due to the changes to inkex and the stock modules, all extensions need to be updated.

This also affects several hundreds of third-party extensions.

The amount of available third-party extensions, as well as users who want to use those and the extension authors would greatly benefit from concise and easy-to-follow upgrade instructions being available.

(in other words: if we don't make it easy for them to upgrade, we may lose a large part of our community ecosystem)

I made a start here:

http://wiki.inkscape.org/wiki/index.php/Updating_your_Extension_for_1.0

but it's far from complete.

Please help fixing those instructions!