Add an extension listing repository

[haubourg]

We now have two available extensions to QWAT

• SIRE extension, embedded in QWAT core
• AquaFri extension provided at https://github.com/qwat/qwat-data-model/pull/263#issuecomment-392697574

We need to reference all available extensions in the documentation and in the extension directory.

Reminder, extensions and customizations can be held inside or outside QWAT core. It depends more on the lifecycle choices between QWAT and the extension, centrlalizing extension in the core link them hardly to QWAT versions.