docs: add NAT traversal related documentation

docs/nat_traversal.md

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+# NAT Traversal
+
+`lnd` has support for NAT traversal using a number of different techniques. At
+the time of writing this documentation, UPnP and NAT-PMP are supported. NAT
+traversal can be enabled through `lnd`'s `--nat` flag.
+
+```shell
+$ lnd ... --nat
+```
+
+On startup, `lnd` will try the different techniques until one is found that's
+supported by your hardware. The underlying dependencies used for these
+techniques rely on using system-specific binaries in order to detect your
+gateway device's address. This is needed because we need to be able to reach the
+gateway device to determine if it supports the specific NAT traversal technique
+currently being tried. Because of this, due to uncommon setups, it is possible
+that these binaries are not found in your system. If this is case, `lnd` will
+exit stating such error.
+
+As a bonus, `lnd` spawns a background thread that automatically detects IP
+address changes and propagates the new address update to the rest of the
+network. This is especially beneficial for users who were provided dynamic IP
+addresses from their internet service provider.