Revise README.md

[gerlero]

As part of openjournals/joss-reviews#7633, make the following proposed changes to the README file.

From #317 (comment):

• README: add statement of need.
• README/REPO: add clear guidelines for third parties wishing to 1) Contribute to the software 2) Report issues or problems with the software 3) Seek support
•  README/REPO: in contributing section, mention how to run the tests locally.

From #318 (comment):

• README: mention hardware used to get the speedup plot.

From #318 (comment):

• REPO/README: Add benchmark code for comparison between foamlib and PyFoam.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.07%. Comparing base (9627c72) to head (bd7f990).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #325   +/-   ##
=======================================
  Coverage   87.07%   87.07%           
=======================================
  Files          15       15           
  Lines        1408     1408           
=======================================
  Hits         1226     1226           
  Misses        182      182           

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[AndreWeiner]

@gerlero I am somehow missing the contribution guidelines and test instructions in this PR, even though they are marked as checked...

[gerlero]

@AndreWeiner sorry, I somehow forgot to make the final push. It should be fixed now.

[AndreWeiner]

@gerlero green light for this PR from my side.