add documentation for exluding whitelistes peer from maxuploadtarget

Author: jonasschnelli

doc/reduce-traffic.md

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+Reduce Traffic
+==============
+
+Some node operators need to deal with bandwidth caps imposed by their ISPs.
+
+By default, bitcoin-core allows up to 125 connections to different peers, 8 of
+which are outbound. You can therefore, have at most 117 inbound connections.
+
+The default settings can result in relatively significant traffic consumption.
+
+Ways to reduce traffic:
+
+## 1. Use `-maxuploadtarget=<MiB per day>`
+
+A major component of the traffic is caused by serving historic blocks to other nodes
+during the initial blocks download phase (syncing up a new node).
+This option can be specified in MiB per day and is turned off by default.
+This is *not* a hard limit; only a threshold to minimize the outbound
+traffic. When the limit is about to be reached, the uploaded data is cut by no
+longer serving historic blocks (blocks older than one week).
+Keep in mind that new nodes require other nodes that are willing to serve
+historic blocks. **The recommended minimum is 144 blocks per day (max. 144MB
+per day)**
+
+Whitelisted peers will never be disconnected, although their traffic counts for
+calculating the target.
+
+## 2. Disable "listening" (`-listen=0`)
+
+Disabling listening will result in fewer nodes connected (remember the maximum of 8
+outbound peers). Fewer nodes will result in less traffic usage as you are relaying
+blocks and transactions to fewer nodes.
+
+## 3. Reduce maximum connections (`-maxconnections=<num>`)
+
+Reducing the maximum connected nodes to a minimum could be desirable if traffic
+limits are tiny. Keep in mind that bitcoin's trustless model works best if you are
+connected to a handful of nodes.