[Docs] Add .env.example for deployment/local setup

[Jayrodri088]

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

PR must include:

• Closes #9 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 expects contributors to use environment variables for deployment/invocation (see deployment commands in Readme.md and the .gitignore exception for .env.example), but .env.example is missing. That slows onboarding and increases setup errors.

Proposed change

Add a .env.example file at repo root with placeholder variables required for Soroban deploy/invoke flows.

Acceptance criteria

• .env.example exists at repo root
• Includes placeholders for at least:
  • Soroban secret key input (or a variable name that scripts can consume)
  • Network selection (testnet/mainnet)
  • Contract IDs for each deployed contract (invoice escrow/token/payment distributor)
  • Any initialization parameters shown in Readme.md (e.g., platform_fee_bps, admin)
• README (or docs) references .env.example in the deployment section
• Values are safe placeholders (no real secrets)

Dependencies

None.

[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.