README: add MacPorts info

[herbygillot]

Adds instructions for installation via MacPorts.

[codecov]

Codecov Report

Merging #762 (f0f8b1d) into master (0f04942) will increase coverage by 1.34%.
The diff coverage is 57.10%.

@@            Coverage Diff             @@
##           master     #762      +/-   ##
==========================================
+ Coverage   49.27%   50.61%   +1.34%     
==========================================
  Files          23       23              
  Lines        2401     2594     +193     
==========================================
+ Hits         1183     1313     +130     
- Misses       1090     1142      +52     
- Partials      128      139      +11     

Impacted Files	Coverage Δ
pkg/container/docker_run.go	1.84% <0.00%> (-0.09%)	⬇️
pkg/model/workflow.go	25.29% <2.43%> (-0.43%)	⬇️
pkg/common/git.go	52.85% <30.15%> (-6.94%)	⬇️
pkg/runner/runner.go	68.96% <33.33%> (-7.51%)	⬇️
pkg/model/planner.go	34.56% <41.37%> (+1.48%)	⬆️
pkg/container/docker_pull.go	36.17% <64.70%> (+17.98%)	⬆️
pkg/runner/step_context.go	74.76% <75.81%> (+5.81%)	⬆️
pkg/runner/run_context.go	80.36% <98.52%> (+3.95%)	⬆️
pkg/runner/command.go	90.58% <100.00%> (+2.35%)	⬆️
pkg/runner/expression.go	87.91% <100.00%> (+1.27%)	⬆️
... and 5 more

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[herbygillot]

Thanks

[herbygillot]

Though maybe the header for that section ( https://github.com/nektos/act#homebrew ) should be '3rd-Party Package Manager' since that section lists a whole lot more than just Homebrew.