Add LLM API key setup guide

- Provide step-by-step instructions for configuring Gemini API key
- Include troubleshooting section for common issues
- Document model configuration options
- Explain benefits of AI review feature
- Make setup process clear for contributors

Author: athikha

.github/LLM_API_KEY_SETUP.md

@@ -0,0 +1,71 @@
+# LLM API Key Setup Guide
+
+This repository uses AI-powered code review through the Presubmit.ai workflow. To enable this feature, you need to configure the `LLM_API_KEY` secret.
+
+## Quick Setup
+
+### Step 1: Get Your Gemini API Key
+
+1. Visit [Google AI Studio](https://makersuite.google.com/app/apikey)
+2. Sign in with your Google account
+3. Click **"Create API Key"**
+4. Copy the generated API key
+
+### Step 2: Configure the Secret in GitHub
+
+1. Go to your repository: `https://github.com/[YOUR_USERNAME]/java-design-patterns`
+2. Click **Settings** → **Secrets and variables** → **Actions**
+3. Click **"New repository secret"**
+4. Set:
+   - **Name**: `LLM_API_KEY`
+   - **Secret**: Your Gemini API key from Step 1
+5. Click **"Add secret"**
+
+### Step 3: Verify Setup
+
+After adding the secret, the next PR or workflow run will automatically use AI review.
+
+## Model Configuration
+
+The workflow is configured to use `gemini-1.5-flash` model. If you need to change this:
+
+1. Edit `.github/workflows/presubmit.yml`
+2. Update the `LLM_MODEL` environment variable
+3. Available models include:
+   - `gemini-1.5-pro`
+   - `gemini-1.5-flash` (default)
+   - `gemini-pro`
+
+## Troubleshooting
+
+### Common Issues
+
+**Error: "LLM_API_KEY secret is not configured"**
+- Solution: Follow Step 2 above to add the secret
+
+**Error: "Model not found" or 404 errors**
+- Check if your API key has access to the specified model
+- Try using `gemini-1.5-pro` instead of `gemini-1.5-flash`
+- Verify your API key is valid and active
+
+**Workflow skips AI review**
+- This is normal behavior when `LLM_API_KEY` is not configured
+- The workflow will continue without AI review
+- Add the secret to enable AI review
+
+### Getting Help
+
+If you encounter issues:
+1. Check the GitHub Actions logs for detailed error messages
+2. Verify your API key is correct and has proper permissions
+3. Ensure you're using a supported model name
+
+## Benefits of AI Review
+
+When properly configured, the AI reviewer will:
+- ✅ Analyze code quality and suggest improvements
+- ✅ Check for potential bugs and security issues
+- ✅ Provide feedback on code style and best practices
+- ✅ Help maintain consistent coding standards
+
+The AI review is **optional** - the main CI/CD pipeline will work without it.