Add documentation about adding panels and dashboards

[eguiraud]

In particular, queries to retrieve datapoints from InfluxDB have some subtleties:

• must use InfluxDB, not rootbench-InfluxDB as source (unless you want old data)
• must select a single nodelabel and build configuration in the WHERE clause
• python benchmarks store runtimes in the duration field, C++ benchmarks use real_time

Oh and you need to press "Save" or lose all your changes :)