Add "Community Tools" section to README

[hummat]

I maintain hloc-cli, a CLI wrapper that simplifies running SfM pipelines with hloc:

hloc /path/to/images --feature superpoint_max --matcher superglue

Handles feature extraction → pairing → matching → reconstruction with sensible defaults.

Proposal: add a "Community Tools" section to the README:

## Community Tools
- [hloc-cli](https://github.com/hummat/hloc-cli) – CLI for SfM pipelines

Currently pins to a specific hloc commit for stability; will track releases as API stabilizes.