docs: Add documentation on how to add new tools to codeact_agent (#5150)

Co-authored-by: openhands <[email protected]>

openhands/agenthub/codeact_agent/README.md

@@ -10,3 +10,57 @@ The conceptual idea is illustrated below. At each turn, the agent can:
    - Execute any valid `Python` code with [an interactive Python interpreter](https://ipython.org/). This is simulated through `bash` command, see plugin system below for more details.
 
 ![image](https://github.com/All-Hands-AI/OpenHands/assets/38853559/92b622e3-72ad-4a61-8f41-8c040b6d5fb3)
+
+## Adding New Tools
+
+The CodeAct agent uses a function calling interface to define tools that the agent can use. Tools are defined in `function_calling.py` using the `ChatCompletionToolParam` class from `litellm`. Each tool consists of:
+
+1. A description string that explains what the tool does and how to use it
+2. A tool definition using `ChatCompletionToolParam` that specifies:
+   - The tool's name
+   - The tool's parameters and their types
+   - Required vs optional parameters
+
+Here's an example of how a tool is defined:
+
+```python
+MyTool = ChatCompletionToolParam(
+    type='function',
+    function=ChatCompletionToolParamFunctionChunk(
+        name='my_tool',
+        description='Description of what the tool does and how to use it',
+        parameters={
+            'type': 'object',
+            'properties': {
+                'param1': {
+                    'type': 'string',
+                    'description': 'Description of parameter 1',
+                },
+                'param2': {
+                    'type': 'integer',
+                    'description': 'Description of parameter 2',
+                },
+            },
+            'required': ['param1'],  # List required parameters here
+        },
+    ),
+)
+```
+
+To add a new tool:
+
+1. Define your tool in `function_calling.py` following the pattern above
+2. Add your tool to the `get_tools()` function in `function_calling.py`
+3. Implement the corresponding action handler in the agent to process the tool's invocation
+
+The agent currently supports several built-in tools:
+- `execute_bash`: Execute bash commands
+- `execute_ipython_cell`: Run Python code in IPython
+- `browser`: Interact with a web browser
+- `str_replace_editor`: Edit files using string replacement
+- `edit_file`: Edit files using LLM-based editing
+
+Tools can be enabled/disabled through configuration parameters:
+- `codeact_enable_browsing`: Enable browser interaction
+- `codeact_enable_jupyter`: Enable IPython code execution
+- `codeact_enable_llm_editor`: Enable LLM-based file editing (if disabled, uses string replacement editor instead)