Add TODO and FIXME notes on some issues

TODO 1: Follow multigraph suggestion from Flameeyes to look into multigraph
plugins (http://munin-monitoring.org/wiki/MultigraphSampleOutput),
in order to reduce the amount of round trips to get the data.

TODO 2: Put warning and critical as vars in config with sensible defaults.

TODO 3: Add additional output options for nvidia-smi only available for
professional GPUs.

FIXME: Possible bug in lines 87-91 of amd_gpu_
Configurable warning and critical temperatures for GPUs
Improve GPU plugins documentation
Merge branch 'pr/401'