added xymon to "Monitoring" section

[Wurzelmann]

added xymon to "Monitoring" section.

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Please take some time to answer the following questions as best you can:

• Why is it awesome?

Simple and versatile monitoring solution, easy to set up and powerful in features/plugins.

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• How many devices/users/services/... do you manage with it?

Professional: approx. 250, personal: approx. 100.

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pro: resource friendly, easy to set up, versatile, easy to write plugins for

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