Update README with installation instructions

[awvwgk]

Installation and integration instructions for fpm and meson.

[codecov]

Codecov Report

Merging #66 (e39ba42) into main (fa4bcbd) will not change coverage.
The diff coverage is n/a.

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  Coverage   88.77%   88.77%           
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  Files           2        2           
  Lines        1221     1221           
  Branches      456      456           
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  Hits         1084     1084           
  Misses         40       40           
  Partials       97       97           

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[certik]

Looks great. Note that on Apple M1, when I install gfortran using conda-forge, the nested functions work in Debug mode, but in Release mode (-O3, etc.) the generated binary segfaults, if the nested function accesses a variable in the parent scope. So the test you added to Meson might not be sufficient, since it doesn't access a variable. I am not a fan of these tests anyway, they seem to slow down the configure step. I think for fpm at least, we should just understand the platform and compiler versions, and know what works and what doesn't, no need to test for it like this (also what about cross-compilation when you cannot even run the binary..).

Anyway, this one is good to merge. The above are just some more general thoughts.

[awvwgk]

Removed the check and added an option to disable the C-API. Our testsuite will verify whether a compiler fully supports our usage of nested functions or not.