doc: Add info how to get libSkiaSharp.dylib

[TopperDEL]

PR Type

What kind of change does this PR introduce?

• Documentation content changes

What is the current behavior?

Building the native dylib for Mac needs an extra step - the build instruction mentions this but on a different page.

What is the new behavior?

This change highlights a bit what needs to be done before building succeeds.

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

Other information

Internal Issue (If applicable):

[spouliot]

This could be a link to the other README.md

[spouliot]

Thanks!

It's done automatically when you build SampleApp.Skia.Generic (or the whole Uno) so I forgot to document it for a standalone compilation.