docs: add shell profile option for API key setup (#3)

* docs: add shell profile option for persistent API key setup

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add link to developer docs for API key generation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: sweetmantech

README.md

@@ -12,12 +12,21 @@ npm install -g @recoupable/cli
 
 ### Authenticate
 
-Set your API key as an environment variable:
+Get your API key from the [Recoup Developer Dashboard](https://developers.recoupable.com/api-reference/introduction#getting-your-api-key), then set it as an environment variable:
 
 ```bash
 export RECOUP_API_KEY=your-api-key
 ```
 
+To persist it across sessions, add it to your shell profile:
+
+```bash
+echo 'export RECOUP_API_KEY=your-api-key' >> ~/.zshrc
+source ~/.zshrc
+```
+
+> Use `~/.bashrc` instead if you use bash.
+
 ### Verify
 
 ```bash