docs: add README note for offline loading guidance

[maod65104-hash]

Summary

• Add a README.md clarification about offline or network-restricted usage for Harmony.
• Explain that users in an offline environment may need the tokenizer vocab file available locally before loading an encoding.
• Keep this scoped as a documentation-only change so the README better communicates offline loading expectations.

Motivation

• Addresses user confusion reflected in Open source FTW #1 and related issue-tracker questions about local tokenizer assets.
• Clarifies that a network-restricted setup may require preparing the vocab file in advance instead of relying on runtime downloads.
• Makes it clearer that README.md is the place to set expectations for docs-only offline loading guidance.

Validation

• Confirmed the README.md note is docs-only and aligned with the current loading behavior.
• Confirmed the text explicitly mentions an offline environment, a network-restricted setup, and the required vocab file.
• Confirmed this is a documentation-only change with no code changes.