updated README.md

A simple example on how to use this package:

```python

import numpy as np

from csst_proto.top_level_interface import flip_image, read_test_image

from csst_proto.top_level_interface import flip_image

from astropy.io import fits

# read an L0 image

| `setup.py`         | sphinx-based documentation      | √√     | √   |     |

## important stages

1. **installation**: `source code` & `setup.py` -> `pip install <package_name>`

2. **a simple guide**: `README.md`

3. **unittest**: `pytest`

4. gitlab CI/CD pipeline: `.gitlab-ci.yml`

5. sphinx-based documentation: `doc/` & `readthedocs.yml`

1. **code**, **code style** & `top_level_interface`

2. **installation**: `source code` & `setup.py` -> `pip install <package_name>`

3. **a simple instruction**: `README.md`

4. **unittest**: `pytest`

5. *gitlab CI/CD pipeline: `.gitlab-ci.yml`

6. *sphinx-based documentation: `doc/` & `readthedocs.yml`

## useful links

- git: https://git-scm.com/

- python packaging user guide: https://packaging.python.org/en/latest/

- packaging binary extensions (C/C++): https://packaging.python.org/en/latest/guides/packaging-binary-extensions

- `setuptools`: https://packaging.python.org/en/latest/tutorials/installing-packages/

- recommended IDEs

  - PyCharm: https://www.jetbrains.com/pycharm/

  - VS Code: https://code.visualstudio.com/

- `git`: https://git-scm.com/

  - Pro Git: https://git-scm.com/book/en/v2

- Numpydoc: https://numpydoc.readthedocs.io/en/latest/format.html

  - for C: https://peps.python.org/pep-0007/

  - for Python: https://peps.python.org/pep-0008/

- unittest: https://docs.python.org/3.8/library/unittest.html

- sphinx: https://www.sphinx-doc.org/en/master/index.html

- `unittest`: https://docs.python.org/3.8/library/unittest.html

- gitlab CI/CD pipeline: https://docs.gitlab.com/ee/ci/pipelines/

- gitlab runners: https://docs.gitlab.com/runner/

- `sphinx`: https://www.sphinx-doc.org/en/master/index.html

- markdown: https://www.markdownguide.org/

- restructured text: https://docutils.sourceforge.io/rst.html

- readthedoc: https://readthedocs.io/ or https://rtfd.io/

- 

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- readthedocs: https://readthedocs.io/ or https://rtfd.io/