Create README for packet_inspector

packet_inspector/src/README.md

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+# What's This?
+
+The packet inspector is a very simple Minecraft proxy for viewing the contents of packets as they are sent/received.
+It uses Valence's protocol facilities to print packet contents.
+This was made for two purposes:
+- Check that packets between Valence and client are matching your expectations.
+- Check that packets between vanilla server and client are parsed correctly by Valence.
+- Understand how the protocol works between the vanilla server and client.
+
+# Usage
+
+First, start a server
+
+```
+cargo r -r --example conway
+```
+In a separate terminal, start the packet inspector. 
+
+```sh
+cargo r -r -p packet_inspector -- 127.0.0.1:25566 127.0.0.1:25565
+```
+
+The client must connect to `localhost:25566`. You should see the packets in `stdout`.
+
+If you only want to see errors, direct `stderr` elsewhere.
+
+```sh
+cargo r -r -p packet_inspector -- 127.0.0.1:25566 127.0.0.1:25565 > log.txt
+```