Add "credential boilerplates" as examples

[OR13]

Developers copy spec examples, we should plan to have better boilerplates in v2, here is an example:

{
  "@context": [
    // "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/ns/credentials/v2",
    // "https://www.w3.org/2018/credentials/examples/v1"
    { "@vocab": "https://www.w3.org/ns/credentials#" }
  ],
  "id": "http://example.edu/credentials/1872",
  "type": ["VerifiableCredential", "NewCredentialType"],
  "issuer": { 
    "id": "did:example:123", 
     "type": ["Organization", "OrganizationType"] 
   },
  "issuanceDate": "2010-01-01T19:23:24Z",
  "credentialSubject": {
    "id": "did:example:456", 
    "type": ["Person", "JobType"],
    "claimName": "claimValue"
  }
}