Add Portal wire readme doc

eth/p2p/portal/README.md

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+# Portal Network Wire Protocol
+## Introduction
+The `eth/p2p/portal` directory holds a Nim implementation of the
+[Portal Network Wire Protocol](https://github.com/ethereum/stateless-ethereum-specs/blob/master/state-network.md#wire-protocol).
+
+Both specification, at above link, and implementations are still WIP.
+
+The protocol builds on top of the Node Discovery v5.1 protocol its `talkreq` and
+`talkresp` messages.
+
+For further information on the Nim implementation of the Node Discovery v5.1
+protocol check out the [discv5](../../../doc/discv5.md) page.
+
+## Test suite
+To run the test suite specifically for the Portal wire protocol, run following
+command:
+```sh
+# Install required modules
+nimble install
+# Run only Portal tests
+nimble test_portal
+```
+
+## portalcli
+This is a small command line application that allows you to run a
+Discovery v5.1 + Portal node.
+
+*Note:* Its objective is only to test the protocol wire component, not to actually
+serve content. This means it will always return empty lists on content requests.
+Perhaps in the future some hardcoded data could added and maybe some test vectors
+can be created in such form.
+
+The `portalcli` application allows you to either run a node, or to specifically
+send one of the message types, wait for the response, and then shut down.
+
+### Example usage
+```sh
+# Install required modules
+# Make sure you have the latest modules, do NOT trust nimble on this.
+nimble install
+# Build portalcli
+nimble build_portalcli
+# See all options
+./eth/p2p/portal/portalcli --help
+# Example command: Ping another node
+./eth/p2p/portal/portalcli ping enr:<base64 encoding of ENR>
+# Example command: Run discovery + portal node
+./eth/p2p/portal/portalcli --log-level:debug --bootnode:enr:<base64 encoding of ENR>