chore: add build-from-source instructions to README install section

[gxmiranda]

Summary

The README install section does not include "build from source" instructions. Other UF components (e.g., gaze) document this path, but replicator omits it, creating an inconsistency.

Context

Users who want to build from source (contributors, users on unsupported platforms, or those who prefer not to use pre-built binaries) have no documented path. The project already has make build and make install targets that handle this, but they are not mentioned in the README.

Fix

Add a "Build from Source" subsection to the README install area with:

1. Prerequisites (Go 1.25+, make)
2. Clone + build commands (git clone, make build or make install)
3. Verification step (replicator version)

Acceptance Criteria

• README includes a "Build from Source" subsection in the install area
• Instructions cover prerequisites, build command, and verification
• Consistent with how other UF components document this step