chore: update readme with auth instructions

[havebeenfitz]

No description provided.

[Copilot]

Pull Request Overview

This PR adds authentication setup instructions to the README, providing developers with detailed steps for configuring App Store Connect API credentials required for the blimp CLI tool.

Key Changes:

• Added new "Authenticating" section documenting required environment variables and setup process
• Reorganized existing documentation by adding "Running" section header and adjusting heading hierarchy
• Included example of private key format and instructions for preparing it for environment variables

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[Copilot]

Corrected spelling of 'whitespacesa' to 'whitespaces'.

Suggested change

	Copy the contents and remove all the whitespacesa and newlines, -----BEGIN PRIVATE KEY----- and -----END PRIVATE KEY-----.
	Copy the contents and remove all the whitespaces and newlines, -----BEGIN PRIVATE KEY----- and -----END PRIVATE KEY-----.

[Copilot]

Corrected spelling of 'authomatically' to 'automatically'.

Suggested change

	Use `export APP_STORE_CONNECT_API_PRIVATE_KEY={bare_key_without_newlines} && ...` locally or create environment variables for your CI provider. JWT token for API client will be created (and recreated on expiration) authomatically if needed variables are in place.
	Use `export APP_STORE_CONNECT_API_PRIVATE_KEY={bare_key_without_newlines} && ...` locally or create environment variables for your CI provider. JWT token for API client will be created (and recreated on expiration) automatically if needed variables are in place.