update README

Author: khillion

README.md

@@ -1,14 +1,33 @@
+## Install
+
 Requires Python 3.6.x or Python 3.7
 
 ``` bash
 git clone https://github.com/common-workflow-language/cwl-utils.git
 virtualenv -p python3.6 venv3.6
 source venv3.6/bin/activate
 pip install cwl-utils
-python docker-extract.py path_to_my_workflow.cwl
 ```
 
-to regenerate install `schema_salad` package and run:
+## Usage
+
+### Pull the image with Docker
+
+This is the default behaviour:
+
+```bash
+python docker_extract.py DIRECTORY path_to_my_workflow.cwl
+```
+
+### Pull the image with Singularity
+
+```bash
+python docker_extract.py --singularity DIRECTORY path_to_my_workflow.cwl
+```
+
+## Regenerate
+
+To regenerate install `schema_salad` package and run:
 
 ```
 schema-salad-tool --codegen python \