Update CLI usage docs to showcase the ability to inject environment variables in shell aliases

docs/cli/usage.mdx

@@ -39,6 +39,22 @@ infisical init
 infisical run -- [your application start command]
 ```
 
+## Injecting environment variables in custom aliases
+Custom aliases can utilize secrets from Infisical. Suppose there is a custom alias `yd` in `custom.sh` that runs `yarn dev` and needs the secrets provided by Infisical.
+```bash
+#!/bin/sh
+
+yd() {
+  yarn dev
+}
+```
+
+To make the secrets available from Infisical to `yd`, you can run the following command:
+
+```bash
+infisical run --command="source custom.sh && yd"
+```
+
 View all available options for `run` command [here](./commands/run)
 
 ## Examples: