update usage -- readme (Project-MONAI#321)

### Status
**Ready**

### Please ensure all the checkboxes:
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applicable items -->
- [x] Codeformat tests passed locally by running `./runtests.sh
--codeformat`.
- [x] In-line docstrings updated.
- [ ] Update `version` and `changelog` in `metadata.json` if changing an
existing bundle.
- [ ] Please ensure the naming rules in config files meet our
requirements (please refer to: `CONTRIBUTING.md`).
- [ ] Ensure versions of packages such as `monai`, `pytorch` and `numpy`
are correct in `metadata.json`.
- [ ] Descriptions should be consistent with the content, such as
`eval_metrics` of the provided weights and TorchScript modules.
- [ ] Files larger than 25MB are excluded and replaced by providing
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---------

Signed-off-by: Wenqi Li <[email protected]>

README.md

@@ -5,7 +5,20 @@ All source code of models (bundles) are tracked in `models/`, and for each disti
 
 ## Getting Started
 
-To download and get started with the models, please see [the model browser](https://monai.io/model-zoo.html) and [the example use cases](https://github.com/Project-MONAI/tutorials/tree/main/model_zoo).
+To browse the available models, please see https://monai.io/model-zoo.
+
+A basic example to download and extract a bundle is:
+
+```bash
+pip install "monai[fire]"
+python -m monai.bundle download "wholeBody_ct_segmentation" --bundle_dir "bundles/"
+```
+
+- The commands will download `wholeBody_ct_segmentation` to the current directory's `bundles/` subdirectory.
+- For more downloading options, please run `python -m monai.bundle download -h`
+- For the specific usage of a bundle, please refer to its `docs` folder, for example, `bundles/wholeBody_ct_segmentation/docs`.
+
+To get started with the models, please see [the example use cases](https://github.com/Project-MONAI/tutorials/tree/main/model_zoo).
 
 ## License