docs(README): Write the Usage section

Signed-off-by: 林博仁(Buo-ren Lin) <buo.ren.lin@gmail.com>

Author: brlin-tw

README.md

@@ -18,6 +18,26 @@ The script extracts:
 * URL links from href attributes of anchor elements
 * Hierarchical structure based on CSS depth indicators (--subitem-depth)
 
+## Usage
+
+Refer to the following instructions to run the conversion script:
+
+1. Install the following runtime dependencies:
+    + [The beautifulsoup4 PyPI package](https://pypi.org/project/beautifulsoup4/)
+1. Copy the HTML snippet of the sidebar into a local file, e.g., `sidebar.html`, refer to the [sample HTML file](gh-actions-docs-sidebar.sample.html) for an example.
+1. Edit the [`html_to_markdown_converter.py`](html_to_markdown_converter.py) script to:
+    + Set the input HTML file path and output Markdown file path.
+    + Edit the `class_` variables according to the actual HTML structure if necessary.
+1. Launch a text terminal.
+1. Change the working directory to where you saved the `html_to_markdown_converter.py` script.
+1. Run the following command to run the utility:
+
+    ```bash
+    python3 html_to_markdown_converter.py
+    ```
+
+   The converted Markdown content will be saved to the specified output file.
+
 ## Licensing
 
 Unless otherwise noted([comment headers](https://reuse.software/spec-3.3/#comment-headers)/[REUSE.toml](https://reuse.software/spec-3.3/#reusetoml)), this product is licensed under [the 3.0 version of the Affero General Public License](https://www.gnu.org/licenses/agpl-3.0.html), or any of its more recent versions of your preference.