[Observium] Example alert checkers, and background information

Hey Guys,

I had some spare time, and decided to write up some more documentation on alert-checkers for observium, and provide a whole bunch of example alerts which could be adapted for your use:

Document: http://www.maartenmoerman.nl/?p=612

I have provided examples for: - Device down - Processor usage is above 80% - Memory usage is above 70% - State sensor is in ALERT state! - Fanspeed is above or under treshold - Traffic exceeds 85% - BGP Session down - Storage exceeds 85% of disk capacity - Port has encountered errors or discards - Port is enabled, but operationally down

Let me know if this is helpful and what you'd like to see added/improved.

Thanks, Maarten