docs: enhance development installation instructions (#286)

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README.md

@@ -110,7 +110,15 @@ For contributors who plan to modify the source code:
 ```bash
 git clone https://github.com/unclecode/crawl4ai.git
 cd crawl4ai
-pip install -e .
+pip install -e .                    # Basic installation in editable mode
+```
+Install optional features:
+```bash
+pip install -e ".[torch]"           # With PyTorch features
+pip install -e ".[transformer]"     # With Transformer features
+pip install -e ".[cosine]"          # With cosine similarity features
+pip install -e ".[sync]"            # With synchronous crawling (Selenium)
+pip install -e ".[all]"             # Install all optional features
 ```
 
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