Poetry build fix + better readme

[noraj]

• pyproject.toml: proper pyproject.toml
• readme: markdown compliant with https://commonmark.org/
• readme: add alternative poetry install steps

Before pyproject.toml update

$ poetry build
Creating virtualenv donpapi-dfVQVXBX-py3.10 in /home/noraj/.cache/pypoetry/virtualenvs
Building donpapi (1.0.0)

$ python -m build --wheel
* Creating virtualenv isolated environment...
* Installing packages in isolated environment... (poetry-core>=1.0.0)
* Getting dependencies for wheel...
* Building wheel...
Traceback (most recent call last):
  File "/usr/lib/python3.10/site-packages/pep517/in_process/_in_process.py", line 351, in <module>
    main()
  File "/usr/lib/python3.10/site-packages/pep517/in_process/_in_process.py", line 333, in main
    json_out['return_val'] = hook(**hook_input['kwargs'])
  File "/usr/lib/python3.10/site-packages/pep517/in_process/_in_process.py", line 249, in build_wheel
    return _build_backend().build_wheel(wheel_directory, config_settings,
  File "/tmp/build-env-lekq9y_k/lib/python3.10/site-packages/poetry/core/masonry/api.py", line 67, in build_wheel
    return WheelBuilder.make_in(poetry, Path(wheel_directory))
  File "/tmp/build-env-lekq9y_k/lib/python3.10/site-packages/poetry/core/masonry/builders/wheel.py", line 74, in make_in
    wb = WheelBuilder(
  File "/tmp/build-env-lekq9y_k/lib/python3.10/site-packages/poetry/core/masonry/builders/wheel.py", line 57, in __init__
    super().__init__(poetry, executable=executable)
  File "/tmp/build-env-lekq9y_k/lib/python3.10/site-packages/poetry/core/masonry/builders/builder.py", line 83, in __init__
    self._module = Module(
  File "/tmp/build-env-lekq9y_k/lib/python3.10/site-packages/poetry/core/masonry/utils/module.py", line 69, in __init__
    raise ModuleOrPackageNotFound(
poetry.core.masonry.utils.module.ModuleOrPackageNotFound: No file/folder found for package donpapi

ERROR Backend subprocess exited when trying to invoke build_wheel

cf. python-poetry/poetry#1110

After pyproject.toml update

$ poetry build           
Building donpapi (1.0.0)
  - Building sdist
  - Built donpapi-1.0.0.tar.gz
  - Building wheel
  - Built donpapi-1.0.0-py3-none-any.whl

$  python -m build --wheel
* Creating virtualenv isolated environment...
* Installing packages in isolated environment... (poetry-core>=1.0.0)
* Getting dependencies for wheel...
* Building wheel...
Successfully built donpapi-1.0.0-py3-none-any.whl