Use static logos in this repository

[giovannipizzi]

As shown e.g. by #11, having only links to external logos is very fragile (people moving the logos in their repo, changing URL, ...) - they won't remember that it's referenced in the registry, and with the final effect of making the registry look like it's broken.

I suggest instead to have a 'static' folder in this repo, and ask people to put the file in there and point with a relative path to files in that folder (file existence can also be checked during the testing phase).

[csadorf]

I think that is a good suggestion and also something that can be easily checked as part of the automated tests.