Create witness.md

Signed-off-by: Ricky Ng-Adam <[email protected]>

docs/witness.md

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+# Witness
+
+anoncreds-rs does validate for witness value if revocation registry and revocation registry id is present:
+
+https://github.com/rngadam/anoncreds-rs/blob/44a22edc49275f4ad0ea805ae9b033d86c22b405/src/data_types/credential.rs#L64-L66
+
+## Proof
+
+in CredentialSignatureProofValue, witness may be none:
+
+https://github.com/rngadam/anoncreds-rs/blob/44a22edc49275f4ad0ea805ae9b033d86c22b405/src/data_types/w3c/proof.rs#L263-L275
+
+Prover recomputes witness here:
+
+https://github.com/rngadam/anoncreds-rs/blob/44a22edc49275f4ad0ea805ae9b033d86c22b405/src/services/prover.rs#L607-L635
+
+## Issuance
+
+the witness is created using CLCredentialIssuer:
+
+https://github.com/rngadam/anoncreds-rs/blob/44a22edc49275f4ad0ea805ae9b033d86c22b405/src/services/issuer.rs#L716-L722
+
+which in turns calls Issuer::sign_credential_with_revoc:
+
+https://github.com/rngadam/anoncreds-rs/blob/44a22edc49275f4ad0ea805ae9b033d86c22b405/src/services/issuer.rs#L814-L829