docs: Describe environment variable configuration in the README.

[hessjcg]

Fixes #2263

[jackwotherspoon]

Suggested change

	the uppercase version of the flag using underscores as word delimiters. For
	example, the `--auto-iam-authn` flag may be set with the environment variable
	`CSQL_PROXY_AUTO_IAM_AUTHN`. An invocation of the Proxy using environment
	variables would look like the following:
	the uppercase version of the flag using underscores as word delimiters.
	For example, the `--auto-iam-authn` flag may be set with the environment variable
	`CSQL_PROXY_AUTO_IAM_AUTHN`.
	An invocation of the Proxy using environment variables would look like the following:

[jackwotherspoon]

let's use either INSTANCE_CONNECTION_NAME or myproject:my-region:myinstance for consistency

Suggested change

	./cloud-sql-proxy my-project:us-central1:my-db-server
	./cloud-sql-proxy <INSTANCE_CONNECTION_NAME>