docs: explain Mux codes

Author: ammario

docs/SUMMARY.md

@@ -30,6 +30,7 @@
 - [Prompting Tips](./prompting-tips.md)
 - [System Prompt](./system-prompt.md)
 - [Telemetry](./telemetry.md)
+- [Mux Codes](./mux-codes.md)
 
 # Development
 

docs/img/mux-gateway-code.webp

@@ -0,0 +1,24 @@
+# Mux Codes
+
+Mux codes are free LLM token credits for evaluating Mux. Coder employees disburse them at events to users we think will enjoy Mux. If you want to get your hands on a code, ask the nearest Coder employee.
+
+A code looks like `ABC-123`.
+
+Redemption is straightforward:
+
+1. Go to [gateway.mux.coder.com](https://gateway.mux.coder.com) and follow instructions to enable the code.
+1. Open Mux (see [install](./install.md) for instructions)
+1. Click the Settings icon in the top left corner (Cmd+, on macOS)
+1. Under Providers, enter the code under Mux Gateway:
+   ![Mux Gateway Code](./img/mux-gateway-code.webp)
+1. When sending a message or creating a workspace, select a `mux-gateway` model:
+   ![Mux Gateway Model](./img/mux-gateway-input.webp)
+   - Optionally, click Star to set the Mux Gateway model as your default model.
+
+That's it! When you run out of credits, you can swap to a different code or
+begin using the model providers directly out-of-pocket.
+
+### Our Ask
+
+As Mux is in its early development stage, we highly value user feedback. Please let us know of any issues you encounter or feature requests in [our
+tracker](https://github.com/coder/mux/issues).