Use env vars and -u in README.md

[nedgar]

Suggested clarification in the README.md:

• Use env vars for ISSUER_API_URL, ISSUER_API_CREDS and VERIFIER_API_URL.
• Use -u "$ISSUER_API_CREDS" instead of -H 'Authorization: Basic ....

[nedgar]

FYI @jimthematrix