Add content for Networks.md

docs/Networks.md

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 # Networks
+
+Bitcore provides support for both the main bitcoin network as well as for
+`testnet3`, the current test blockchain. We encourage the use of
+`Networks.livenet` and `Networks.testnet` as constants. Note that the library
+sometimes may check for equality against this object. Avoid creating a deep
+copy of this object and using that.
+
+## Setting the default network
+
+Most project will only need to work in one of either networks. The value of
+`Networks.defaultNetwork` can be set to `Networks.testnet` if the project will
+needs only to work on testnet (the default is `Networks.livenet`).
+
+## Network constants
+
+The functionality of testnet and livenet is mostly similar (except for some
+relaxed block validation rules on testnet). They differ in the constants being
+used for human representation of base58 encoded strings. These are sometimes
+referred to as "version" constants.
+
+## Source
+TODO: Include source here