Update README.md (#15926)

docs/README.md

@@ -343,7 +343,13 @@ contains the example docstring to the [documentation_tests.txt](../utils/documen
 
 ### For Python files
 
-You can run all the tests in the docstrings of a given file with the following command, here is how we test the modeling file of Wav2Vec2 for instance:
+You will first need to run the following command (from the root of the repository) to prepare the doc file (doc-testing needs to add additional lines that we don't include in the doc source files):
+
+```bash
+python utils/prepare_for_doc_test.py src docs
+```
+
+Then you can run all the tests in the docstrings of a given file with the following command, here is how we test the modeling file of Wav2Vec2 for instance:
 
 ```bash
 pytest --doctest-modules src/transformers/models/wav2vec2/modeling_wav2vec2.py -sv --doctest-continue-on-failure
@@ -355,6 +361,12 @@ If you want to isolate a specific docstring, just add `::` after the file name t
 pytest --doctest-modules src/transformers/models/wav2vec2/modeling_wav2vec2.py::transformers.models.wav2vec2.modeling_wav2vec2.Wav2Vec2ForCTC.forward -sv --doctest-continue-on-failure
 ```
 
+Once you're done, you can run the following command (still from the root of the repository) to undo the changes made by the first command before committing:
+
+```bash
+python utils/prepare_for_doc_test.py src docs --remove_new_line
+```
+
 ### For Markdown files
 
 You will first need to run the following command (from the root of the repository) to prepare the doc file (doc-testing needs to add additional lines that we don't include in the doc source files):