add default long description text for emmet-core

[tsmathis]

Trying to pip install emmet-core in a container (Docker) context will throw the following error:

× python setup.py egg_info did not run successfully.
│ exit code: 1
╰─> [7 lines of output]
    Traceback (most recent call last):
      File "<string>", line 2, in <module>
      File "<pip-setuptools-caller>", line 34, in <module>
      File "/tmp/pip-install-hxr58vdl/emmet-core_4bad4006d3d442a6ae60dc59dcf5ac78/setup.py", line 10, in <module>
        long_description=open("../README.md").read(),  # noqa: SIM115
                         ^^^^^^^^^^^^^^^^^^^^
    FileNotFoundError: [Errno 2] No such file or directory: '../README.md'
    [end of output]

PR adds some default text for the long_description field to allow container builds to succeed.

[tsmathis]

tests failing again due to #389 in pmg-analysis-diffusion. No new release over there yet.