Update usage.md

Update usage documentation on readthedoc

Author: killianrochet

docs/usage.md

@@ -6,4 +6,14 @@ To download the metadata of a project you can use the command line tool `RedCapB
 
 ### Building projects
 
-The functions for project building, validation and control are only accessible via Python. Read the module documentation to learn more about these methods.
+The functions for project building, validation and control are only accessible via Python. Read the module documentation to learn more about these methods.
+
+### Download Function
+
+To download your experiments from eLab, follow the procedure below.
+
+1. After completing the **installation** module, go to your terminal
+2. Enter the following command:
+`ElabBridge extended_download output_file_path config_file_path tag -f csv `
+3. `output_file_path` is the **complete** path to your csv file where your experiments will be saved
+4. `config_file_path` is the path to your `.json` file containing your API key as seen above