docs: add profile storage location to README (#252)

- Added clear documentation of where profiles are stored on each platform
- Linux/macOS: ~/.config/redisctl/config.toml
- Windows: %APPDATA%\redis\redisctl\config.toml
- Helps users understand where their configuration is saved

Part of #251

Author: joshrotenberg

README.md

@@ -57,6 +57,10 @@ redisctl profile set cloud-prod \
 redisctl --profile cloud-prod cloud database list
 ```
 
+Profiles are stored in:
+- **Linux/macOS**: `~/.config/redisctl/config.toml`
+- **Windows**: `%APPDATA%\redis\redisctl\config.toml`
+
 ### 2. Basic Commands
 
 ```bash