Add readme to the certs folder

sdk/tests/fixtures/certs/README.md

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+# Sample Certificates
+
+This folder contains certificates and signing keys that are used by unit tests and sample applications.  The certificates are not intended for production use.  
+
+The certificates are organized by supported C2PA signing algorithm.  For each supported signing algorithm (ps256, ps384, ps512, es256, es384, Ees512, ed25519) there are three files.
+* {alg}.pem - private signing key in PEM format
+* {alg}.pub - certificate chain from signing certificate to the last certificate before the root CA, as a concatenated list of certficates
+* {alg}_root.pub_key - public key of the root CA used to verify the last certificate in the certificate chain.
+
+## More Info
+
+For more information on digital signatures and the C2PA trust model see  [https://c2pa.org/specifications/specifications/1.0/specs/C2PA_Specification.html#_digital_signatures] and [https://c2pa.org/specifications/specifications/1.0/specs/C2PA_Specification.html#_credential_types]