Add c-lightning exporter to exporters.md (#1737)
Add c-lightning exporter to exporters.md (#1737)

The exporter was already added to the port-reservations list, and exports a variety of metrics from the Lightning Network client.

Signed-off-by: Christian Decker <bd5e2b43faa09124ca3857e2b70edbfebff5e94e@gmail.com>
prometheus/docs,prometheus/docs,prometheus/docs