[doc] add documentation how to reduce traffic

doc/reducetraffic.md

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+REDUCE TRAFFIC
+==============
+
+Some node operators need to deal with bandwith cap given by their ISPs.
+
+By default, bitcoin-core allows up to 125 connections to different peers, 8 of
+them outbound (and therefore 117 max inbound connections).
+
+The default settings can result in relatively significant traffic consumption.
+
+
+Ways to reduce traffic:
+
+1. Use `-maxuploadtarget=<MiB per day>`
+
+A major part of the traffic is caused by serving historic blocks to other nodes
+in initial blocks download state (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 but a threshold to minimize the outbound
+traffic. When the limit is about to be reached, the uploaded data is cut by not
+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)**
+
+2. Disable "listening" (`-listen=0`)
+
+Disable listening will result in fewer nodes connected (remind the max of 8
+outbound peers). Fewer nodes will result in less traffic usage because relaying
+blocks and transaction needs to be passed to fewer nodes.
+
+3. Reduce maximal connections (`-maxconnections=<num>`)
+
+Reducing the connected nodes to a miniumum can be desired in case traffic
+limits are tiny. Keep in mind that bitcoin trustless model works best if you are
+connected to a handfull of nodes.