Add Snowflake Cortex Code installation instructions (#21)

## Summary
- Adds installation instructions for Snowflake Cortex (Cortex Code) to
the README
- Includes both full repo install (`cortex skill add
streamlit/agent-skills`) and sparse checkout for just the main skill

## Test plan
- [x] Verify `cortex skill add streamlit/agent-skills` works
- [x] Confirm README renders correctly on GitHub

.... Generated with [Cortex Code](https://go/cortex-cli)

---------

Co-authored-by: Zachary Blackwood <zachary.blackwood@snowflake.com>
Co-authored-by: Cortex Code <noreply@snowflake.com>

Author: 3 people

README.md

@@ -61,6 +61,14 @@ cp -r developing-with-streamlit ~/.cursor/skills/
 
 Or add skills directly to your project's `.cursor/skills/` directory.
 
+### Snowflake Cortex Code
+
+Install the skill directly from GitHub:
+
+```bash
+cortex skill add streamlit/agent-skills
+```
+
 ### Other AI Assistants
 
 | Agent | Skills Folder | Documentation |