docs: list Labkit::ApplicationSli in the README

Bob Van Landuytrequested to merge
bvl/doc-application-sli into master

What does this MR do?

When Gitlab::Metrics::Sli was moved into labkit-ruby as Labkit::ApplicationSli, the top-level README.md Functionality list was never updated to mention it.

This MR adds an entry for Labkit::ApplicationSli next to Labkit::UserExperienceSli, linking to the module's own README so the component is discoverable alongside the others.

Related

Closes #52 (closed)

🤖 beep boop

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