Add information on FreeBSD port

[awvwgk]

https://www.freshports.org/science/dftd4

[codecov]

Codecov Report

Merging #142 (15e486a) into main (982bb90) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
- Coverage   59.08%   59.02%   -0.07%     
==========================================
  Files          42       42              
  Lines        2977     3014      +37     
  Branches     1016     1019       +3     
==========================================
+ Hits         1759     1779      +20     
- Misses        640      654      +14     
- Partials      578      581       +3     

Impacted Files	Coverage Δ
python/dftd4/parameters.py	72.72% <0.00%> (-5.66%)	⬇️
python/dftd4/interface.py	96.87% <0.00%> (-0.04%)	⬇️
python/dftd4/ase.py	76.00% <0.00%> (ø)
src/dftd4/data/r4r2.f90	37.50% <0.00%> (ø)
python/dftd4/qcschema.py	96.49% <0.00%> (ø)
test/unit/test_param.f90	50.00% <0.00%> (ø)
python/dftd4/test_qcschema.py	93.05% <0.00%> (ø)
python/dftd4/test_interface.py	100.00% <0.00%> (ø)
python/dftd4/test_parameters.py	100.00% <0.00%> (ø)
python/dftd4/test_libdftd4.py
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 982bb90...15e486a. Read the comment docs.

[yurivict]

@awvwgk

The recommended way to install software on FreeBSD is through packages. The command is: pkg install dftd4.

You may want to add this command before instructions how it can also be installed from the port.