Nasty bugs & Contribution guidelines

⭐️ Features

• Add Contribution guidelines (at last !)

🐞 Bug fixes

• Add x-death header to dead-lettered messages (thanks to @gr423)
• Redelivered messages are now marked as such (thanks to @pawelebe)
• Fix consumer cancelation when another connection is closed (was needed by akka-stream-alpakka-amqp, thanks for @mdiasribeiro for providing a test case covering the issue)

🔨 Dependency upgrades

• Upgrade to amqp-client 5.7.3
• Replace Cobertura (deprecated) by JaCoCo (preparing Java9+ compatibility)