Add Thessaloniki GDCR event
Add Thessaloniki GDCR event

svenlange/coderetreat.github.io,yugangbai/coderetreat.github.io,svenlange/coderetreat.github.io,coderetreat/coderetreat.github.io,yugangbai/coderetreat.github.io,coderetreat/coderetreat.github.io,svenlange/coderetreat.github.io,coderetreat/coderetreat.github.io,coderetreat/coderetreat.github.io,yugangbai/coderetreat.github.io,yugangbai/coderetreat.github.io,svenlange/coderetreat.github.io