Add caution for polling nodes (#46)

Add a note to caution operators about frequently polling or querying 
groups of mesh nodes, which can degrade mesh performance.

arednServicesGuide/services_overview.rst

@@ -8,6 +8,8 @@ The purpose for this section of the AREDN |trade| documentation is to identify t
 
 * An important consideration for selecting programs is to understand the impact each service will have on the performance and reliability of the network during the times when digital communication is required. As a best practice, choose programs which require the least amount of computing and network resources in order to operate successfully.
 
+.. note:: The consideration above is especially important if you are deploying a service which regularly queries other nodes across the network. For example, if you deploy a network management system which polls metrics from remote mesh nodes, you need to carefully consider how many metrics you poll and how often you request them. Realize that polling dozens of metrics from each node every few seconds is likely to degrade mesh performance. Be sure to let node owners know what you are planning to do and get their permission/agreement for your polling schedule.
+
 * It is equally important to choose data services that meet the criteria defined in FCC Part 97 regulations for amateur radio services. Try to avoid programs that use encryption or proprietary compression algorithms, which may be interpreted as "encoding messages for the purpose of obscuring their meaning."
 
 * As a general rule services should be run on separate LAN-connected computers rather than on the AREDN |trade| node itself. Radio nodes have very limited resources which should be conserved for node operation rather than running extra programs. Try to select external computers that have low power requirements, since many AREDN |trade| deployments are off-grid and without any external network access. Many operators use `Raspberry Pi <https://en.wikipedia.org/wiki/Raspberry_Pi>`_ computers which are small, easy to transport, and require minimal DC power for operation.