docs: add Searchd to Specialized GEO Agencies

[junhodo]

Summary

Add Searchd to the Specialized GEO Agencies section. Searchd focuses on answer engine optimization for Asian consumer brands selling into the US, UK, and Japan.

Disclosure: I am Bale from Searchd and am submitting our own agency for review.

Changes

• Add one neutral description and the official website link.
• Preserve the existing format and alphabetical placement, between RevenueZen and Siege Media.

Source: https://searchd.ai/
Methodology: https://searchd.ai/how-it-works

The methodology page describes fixed buyer-question panels, per-engine measurement, technical and content improvements, and third-party-source work. Searchd is early-stage; illustrative examples are not client results, and this submission makes no performance or ranking claims.

Verification

• git diff --check passed; the diff adds one line to readme.md.
• Both the official website and methodology link returned HTTP 200.
• Documentation-only change; this repository has no test or build configuration. Local language-server diagnostics were unavailable because the LSP daemon could not be reached.