[Docs] Rename Readme.md to README.md

[Jayrodri088]

Complexity / points: Trivial (100 points) - maintainer label: points-100-trivial

PR must include:

• Closes #15 in the PR description
• Tests added or updated where behavior changes
• Docs updated if public API, lifecycle, or deploy steps change
• No unwrap() in production Soroban contract paths (project convention)
• cargo test passes locally; match CI (cargo fmt, cargo clippy -D warnings) when touching Rust

Problem

The repo root README file is named Readme.md (capital R, lowercase rest). Many tools and conventions expect README.md specifically.

On case-sensitive environments (some CI systems, package managers, docs tooling), the README may not render or may be missed.

Proposed change

Rename Readme.md to README.md and verify that links inside still work.

Acceptance criteria

• Readme.md renamed to README.md
• GitHub renders the README on the repo root
• Any internal relative links are corrected if they were case-sensitive
• Link to docs/API.md remains correct after docs/API work is completed

Dependencies

Recommended to keep this independent, but it can be coordinated with [Docs] Add missing docs/API.md... so that all README links are valid as soon as possible.

[Jayrodri088]

Deferred (Wave 1 scope)
Maintainers are focusing this wave on implementation: escrow, invoice-token lifecycle, settlement/payment flows, payment-distributor, tests, and security bugs — aligned with decentralized invoice factoring on Stellar (liquidity for sellers, yield for investors, on-chain escrow/settlement).

Documentation issues will be reopened or recreated in a later wave. If you already started work, comment here and we can link a follow-up issue.