docs: add configure docs

README.md

@@ -149,8 +149,9 @@ curl --connect-to my-endpoint.example.com:8000:localhost:8000 http://my-endpoint
 ## Docs
 
 See [docs](./docs) for details on deploying and managing Pico:
-  - [Kubernetes](./docs/deploy/kubernetes.md)
-  - [Observability](./docs/deploy/observability.md)
+- [Configure](./docs/deploy/configure.md)
+- [Kubernetes](./docs/deploy/kubernetes.md)
+- [Observability](./docs/deploy/observability.md)
 
 ## Limitations
 

docs/deploy/configure.md

@@ -0,0 +1,14 @@
+# Configure
+
+Pico server and agent support both YAML configuration and command-line flags.
+
+The YAML file path can be set using `--config.path`.
+
+## Variable Substitution
+
+When enabling `--config.expand-env`, Pico will expand environment variables
+in the loaded YAML configuration. This will replace references to `${VAR}`
+and `$VAR` with the corresponding environment variable.
+
+If the environment variable is not defined, it will be replaced with an empty
+string. You can also defined a default value using form `${VAR:default}`.