docs: Add required plugin docs (#14)

Author: AngelOnFira

Assets/Rivet/README.md

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+# Rivet Unity Plugin
+
+The Rivet Unity Plugin allows you to connect your game with Rivet, and easily
+build and ship your multiplayer games to the world in under a minute.
+
+More learning content related to Unity and Rivet can be found in the [Rivet
+Learning Center](https://rivet.gg/learn/unity). You can also find an example of
+this plugin being used in the [Rivet examples
+repository](https://github.com/rivet-gg/examples/tree/main/unity/tanks-fishnet).
+
+# 1.0 Table of Contents
+
+- [Rivet Unity Plugin](#rivet-unity-plugin)
+- [1.0 Table of Contents](#10-table-of-contents)
+- [2.0 Plugin Usage](#20-plugin-usage)
+  - [2.1 Linking](#21-linking)
+  - [2.2 Playtest Tab](#22-playtest-tab)
+  - [2.3 Deploy Tab](#23-deploy-tab)
+  - [2.4 Settings Tab](#24-settings-tab)
+  - [2.5 API Usage and Reference](#25-api-usage-and-reference)
+
+# 2.0 Plugin Usage
+
+## 2.1 Linking
+
+First, you'll need to link the Rivet Unity Plugin to your project on the Rivet
+Hub. You can open the Rivet Unity Plugin window by going to `Window > Rivet
+Plugin`.
+
+At this point, if the Rivet CLI isn't installed, the plugin shows a button that
+handles the installation for you.
+
+After the installation, you'll be prompted to link your project with the Rivet
+Hub. By clicking "Sign in with Rivet", a browser window will be opened and let
+you select or create a project on the Rivet Hub.
+
+## 2.2 Playtest Tab
+
+The playtesting tab allows you to select how your local game and game builds
+will connect to Rivet servers. There are two items you can change here:
+
+- **Server**: This allows you to select if you want to connect to a locally
+  hosted server with `This machine`, or Rivet servers with `Rivet servers`. More
+  often, you'll likely want to use Rivet servers so that you can quickly get
+  others testing the game with publically accessible lobbies. Local testing can
+  allow you to run a server on your machine for better debugging purposes.
+- **Namespace**: This will allow you to choose which namespace you'd like to
+  connect to. By default, the options are "Production" and "Staging", but others
+  can be added on the Rivet Hub.
+
+## 2.3 Deploy Tab
+
+The Deploy tab allows you to build your game server and upload it to Rivet. You
+can choose which namespace you'd like to deploy to. By default, the options are
+"Production" and "Staging".
+
+Staging can be used for testing, or to playtest with others. After you upload to
+staging, make sure to go back to the Playtest tab and select the "Staging"
+namespace, and change the server to "Rivet Servers".
+
+Once you're ready to deploy to production, you can select the "Production"
+namespace and deploy your server there. As soon as you deploy to production,
+your updated server will be live for any future lobbies that are started.
+
+## 2.4 Settings Tab
+
+From the settings tab, you may unlink your game. This will bring you back to the
+login screen, allowing you to re-link your project if needed.
+
+## 2.5 API Usage and Reference
+
+You can find the full Rivet API reference
+[here](https://rivet.gg/docs/matchmaker). Items under the Matchmaker API
+(`lobbies`, `players`, and `regions`) are all implemented in the Unity plugin.
+Here's how you might use the `matchmaker/lobbies/find` endpoint:
+
+```csharp
+// Find lobby and connect with FishNet
+var rivetManager = FindObjectOfType<RivetManager>();
+StartCoroutine(rivetManager.FindLobby(new FindLobbyRequest
+{
+    GameModes = new[] { gameMode },
+}, res =>
+{
+    // Connect to server
+    var port = res.Ports["default"];
+    Debug.Log("Connecting to " + port.Hostname + ":" + port.Port);
+    var networkManager = FindObjectOfType<NetworkManager>();
+    networkManager.ClientManager.StartConnection(port.Hostname, port.Port);
+
+    UpdateConnectionInfo();
+}, fail => { Debug.Log($"Failed to find lobby: {fail}"); }));
+```