Add note on making CMPLXFOIL super fast to build instructions

[A-CGray]

Purpose

I found that I got a ~2x speedup from replacing -O2 with -Ofast -march=native in my build config and the regression tests against reference values still passed. I think we probably don't want to make these the default flags because Ofast in unsafe and march=native might mess with the conda build but I added a note to the docs for people who are building CMPLXFOIL locally themselves.

Expected time until merged

1 day

Type of change

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (non-backwards-compatible fix or feature)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no API changes)
• [x] Documentation update
• [ ] Maintenance update
• [ ] Other (please describe)

Testing

Checklist

• [ ] I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• [ ] I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• [ ] I have run unit and regression tests which pass locally with my changes
• [ ] I have added new tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation

[codecov[bot]]

Codecov Report

Merging #22 (357ac91) into main (64b7522) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   78.50%   78.50%           
=======================================
  Files           4        4           
  Lines         563      563           
=======================================
  Hits          442      442           
  Misses        121      121           

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