Add accounts.key to README

[ma3u]

What this PR changes/adds

Add accounts.key to README during the start of the Identity Hub

Why it does that

Error message

java -Dweb.http.presentation.port=10001 \
     -Dweb.http.presentation.path="/api/presentation" \
     -Dweb.http.port=8181 \
     -Dweb.http.path="/api" \
     -Dweb.http.identity.port=8182 \
     -Dweb.http.identity.path="/api/identity" \
     -Dedc.ih.api.superuser.key="c3VwZXItdXNlcgo=c3VwZXItc2VjcmV0Cg==" \
     -jar launcher/identityhub/build/libs/identity-hub.jar

Fix error by start by given an additional API key:

  -Dedc.api.accounts.key="bmV3LXVzZXI=\nbmV3LXNlY3JldA=="\

[github-actions]

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[ma3u]

done.