Update README with Skills.sh installation guide

[fellyph]

Added installation instructions for Skills.sh to the README and moved how it works before the installation process.

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[brandonpayton]

@fellyph we are also recommending installation via our own custom shared/scripts/skillpack-install.mjs script below in addition to recommending npx skills here.

I'd personally prefer using a general purpose command for installation that is maintained by the community since it is common logic for those installing skill sets. But either way, it seems like it would be good to pick one.

Is there something the custom scripts do that npx skills doesn't do? If not, I think we should check with the AI team about whether they'd be up for switching to npx skills.

I think the custom script has been there from the beginning when @Jameswlepage hard-reset this repo and contributed a large set of WP agent skills (from under the Automattic GH org).

[fellyph]

I think we get more people installing via npx skills, a better position on popular skills, and more visibility.