docs(configuration): document env variables and add .env.example

[Jagadeeshftw]

Summary

Documents the project's environment configuration. Docs-only — no code changes.

• Adds a Configuration section to README.md with a variable | purpose | default | example table covering the only two variables the code reads:
  • PORT (read in src/index.ts and src/server.ts, default 3001).
  • NODE_ENV (its test value disables the rate limiter and per-request logging).
    No other variables are invented.
• Adds .env.example as the template, with an explanatory comment per line and safe placeholder defaults.
• Notes that .env.example is the template and .env is git-ignored (confirmed against .gitignore), and cross-links the section from the existing "Setup (contributors)" steps.

npx tsc --noEmit passes (no code change).

Rendered table

Variable	Purpose	Default	Example
PORT	TCP port the HTTP server binds to.	3001	8080
NODE_ENV	Runtime mode; test disables the rate limiter and per-request logging.	(unset)	production

Closes #85

Security notes

.env.example ships only non-secret placeholder defaults and the docs reinforce that the real .env is git-ignored, keeping secrets out of version control.

[mikewheeleer]

good, documenting the env vars closes a real gap. lgtm