[README] document notebook conversion (#480)

Author: mishig25

README.md

@@ -8,6 +8,7 @@ This is the package we use to build the documentation of our Hugging Face repos.
   * [Installation](#installation)
   * [Previewing](#previewing)
   * [Doc building](#doc-building)
+  * [Writing in notebooks](#writing-in-notebooks)
   * [Templates for GitHub Actions](#templates-for-github-actions)
     + [Enabling multilingual documentation](#enabling-multilingual-documentation)
     + [Redirects](#redirects)
@@ -92,6 +93,21 @@ which will build HTML files in `~/tmp/test-build`. You can then inspect those fi
 - add `[[open-in-colab]]` in the tutorial for which you want to build a notebook
 - add `--notebook_dir {path_to_notebook_folder}` to the build command.
 
+## Writing in notebooks
+
+You can write your docs in jupyter notebooks & use doc-builder to: turn jupyter notebooks into mdx files.
+
+In some situations, such as course & tutorials, it makes more sense to write in jupyter notebooks (& use doc-builder converter) rather than writing in mdx files directly.
+
+The process is:
+1. In your `build_main_documentation.yml` & `build_pr_documentation.yml` enable the flag [convert_notebooks: true](https://github.com/huggingface/doc-builder/blob/main/.github/workflows/build_main_documentation.yml#L46-L48). 
+2. After this flag is enabled, doc-builder will convert all .ipynb files in [path_to_docs](https://github.com/huggingface/doc-builder/blob/main/.github/workflows/build_main_documentation.yml#L19-L20) to mdx files.
+
+Moreover, you can locally convert .ipynb files into mdx files.
+```bash
+doc-builder notebook-to-mdx {path to notebook file or folder containing notebook files}
+```
+
 ## Templates for GitHub Actions
 
 `doc-builder` provides templates for GitHub Actions, so you can build your documentation with every pull request, push to some branch etc. To use them in your project, simply create the following three files in the `.github/workflows/` directory: