doc(readme): typos and missing key in .env.template

[gali-leilei]

Describe this PR

• fix missing instruction, environmental_variable and typo when onboarding new user with readme.md.

Checklist for PR

Must Do

• Write a good PR title and description, i.e. feat(agent): add pdf tool via mcp, perf: make llm client async and fix(utils): load custom config via importlib etc. CI job check-pr-title enforces Angular commit message format to PR title.
• Run make precommit locally. CI job lint enforce ruff default format/lint rules on all new codes.
• Run make pytest. Check test summary (located at report.html) and coverage report (located at htmlcov/index.html) on new codes.

Nice To Have

• (Optional) Write/update tests under /tests for feat and test PR.
• (Optional) Write/update docs under /docs for docs and ci PR.

[RogerChern]

LGTM