Update references to README.rst to README.md

[mathomp4]

In trying to install ffnet from master here today I got hit with:

Collecting git+https://github.com/mrkwjc/ffnet
  Cloning https://github.com/mrkwjc/ffnet to ./tmp-for-ffnet/pip-req-build-d4yhl63a
  Running command git clone --filter=blob:none --quiet https://github.com/mrkwjc/ffnet /gpfsm/dswdev/mathomp4/GEOSpyD/tmp-for-ffnet/pip-req-build-d4yhl63a
  Resolved https://github.com/mrkwjc/ffnet to commit a0fd03bb2771b978c414abd6dc04857740f25789
  Preparing metadata (setup.py): started
  Preparing metadata (setup.py): finished with status 'error'
  error: subprocess-exited-with-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 "/gpfsm/dswdev/mathomp4/GEOSpyD/tmp-for-ffnet/pip-req-build-d4yhl63a/setup.py", line 69, in <module>
          long_description  = open('README.rst', 'r').read(),
                              ^^^^^^^^^^^^^^^^^^^^^^^
      FileNotFoundError: [Errno 2] No such file or directory: 'README.rst'
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.

note: This is an issue with the package mentioned above, not pip.
hint: See above for details.

The fix seems simple enough as README.rst is now README.md, so I changed the two README.rst mentions I saw in the code to README.md

[mathomp4]

Huh. Did I somehow break the CI with this change?

[mrkwjc]

You fixed things :) Thanks!