Add readme for the multi selection component

[JustinyAhin]

Description

Added documentation for the multi selector inspector component (see #22891)

How has this been tested?

N/A, documentation only

Types of changes

Documentation

Checklist:

• [n/a] My code is tested.
• [n/a] My code follows the WordPress code style.
• [n/a] My code follows the accessibility standards.
• [n/a] My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[JustinyAhin]

@mkaz for the code sample, I'm wondering if I should rather do something like:

const SelectedBlockCount = getSelectedBlockCount();
if ( SelectedBlockCount > 1 ) {
    const MyMultiSelectionInspector = () => <MultiSelectionInspector />;
}

Maybe looks more clear?

[mkaz]

I think you missed a line to replace from your template.

Also your code example in the comment is better, maybe prefix with a description of "an example detecting multiple blocks to determine the component used" or something like that.

[mkaz]

👍