Add steps for installation using MacPorts

[manojkarthick]

Hi! I'm a macOS user and I noticed that this tool is available on Homebrew but was missing from MacPorts. So I took the liberty of adding the package. This PR updates the README with installation steps.

Relevant links:

• MacPorts: https://www.macports.org/
• Installation Config: https://github.com/macports/macports-ports/blob/master/net/s5cmd/Portfile

[sonmezonur]

@manojkarthick Hello! Thank you for your contribution.

[igungor]

Our release process (goreleaser under the hood) automatically creates tarballs, updates Docker images and brew formula. Unfortunately it doesn't support macports. That's why it's missing from the readme. Also the fact that the whole team uses brew :)

I don't know much about macports but I see that s5cmd is in the index, so the Portfile needs to be updated after each release, right? I'm not sure we'd do that. If we merge this, we should mention that macports is not officially supported, so updates may fall behind.

[igungor]

If we merge this, we should mention that macports is not officially supported, so updates may fall behind.

Hi @manojkarthick. Is the above approach sound good to you? If so, would you update the readme please?

[manojkarthick]

If we merge this, we should mention that macports is not officially supported, so updates may fall behind.

Hi @manojkarthick. Is the above approach sound good to you? If so, would you update the readme please?

Sounds good @igungor - I've updated the PR with a note on support.

[igungor]

Thanks!