Update env vars in the readme

[Cleop]

As a developer
I want to have up to date information about the project's env vars in the readme
So that I can set up and run the project locally easily.

The env vars in the readme are out of date and need updating.

• Update the readme with the names of the new env vars.

export SECRET_KEY_BASE=
export GITHUB_CLIENT_ID=
export GITHUB_CLIENT_SECRET=
export GITHUB_ACCESS_TOKEN_OLD=
export GITHUB_ACCESS_TOKEN=
export DATABASE_URL=
export PORT=