docs: apply for OpenSSF Best Practices badge

[andrewgazelka]

Problem

OpenSSF Scorecard reports CII-Best-Practices 0/10 because the repo has no OpenSSF (formerly CII) Best Practices badge.

Context

The badge is a self-certification at https://www.bestpractices.dev/. Filling out the silver/passing form gets the project a public score and an embeddable badge, and Scorecard credits the badge automatically once the listing is linked back to the repo.

Desired outcome

1. Register github.com/indexable-inc/index at bestpractices.dev.
2. Complete at least the "passing" tier (most items the repo already satisfies: MIT license, security policy, signed commits via GitHub, public issue tracker, pinned deps).
3. Add the badge markdown to README.md.

This is a human action (form-filling + email verification); no commit can do it.