fix prints to stdout and stderr and update README.md (#77)

Signed-off-by: Asra Ali <asraa@google.com>

README.md

@@ -78,8 +78,12 @@ verified SLSA provenance produced at
         "trigger": "workflow_dispatch",
         "issuer": "https://token.actions.githubusercontent.com"
 }
+{"_type":"https://in-toto.io/Statement/v0.1","predicateType":"https://slsa.dev/provenance/v0.2","subject":[{"name":"binary-linux-amd64","digest":{"sha256":"723ccb85318bc8b1a9dd29340612ce1268cd3418d70f68e775edbdc16d1d9158"}}],"predicate":{...}}
 successfully verified SLSA provenance
 ```
+
+The verified in-toto statement is written to stdout and can be used to pipe into policy engines. 
+
 ## Technical design
 
 ### Blog post

main.go

@@ -100,7 +100,7 @@ func verify(ctx context.Context,
 	if err != nil {
 		return fmt.Errorf("%w: %s", pkg.ErrorInvalidDssePayload, "decoding payload")
 	}
-	fmt.Fprintf(os.Stdout, string(pyld))
+	fmt.Fprintf(os.Stdout, "%s\n", string(pyld))
 	return nil
 }
 
@@ -138,7 +138,7 @@ func main() {
 		os.Exit(2)
 	}
 
-	fmt.Fprintf(os.Stderr, "successfully verified SLSA provenance")
+	fmt.Fprintf(os.Stderr, "successfully verified SLSA provenance\n")
 }
 
 func isFlagPassed(name string) bool {

pkg/provenance.go

@@ -11,6 +11,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"os"
 	"strings"
 	"time"
 
@@ -315,7 +316,6 @@ func FindSigningCertificate(ctx context.Context, uuids []string, dssePayload dss
 	for _, uuid := range uuids {
 		entry, err := verifyTlogEntry(ctx, rClient, uuid)
 		if err != nil {
-			fmt.Printf(err.Error())
 			continue
 		}
 		cert, err := extractCert(entry)
@@ -340,7 +340,7 @@ func FindSigningCertificate(ctx context.Context, uuids []string, dssePayload dss
 			continue
 		}
 		// success!
-		fmt.Printf("Verified against tlog entry %d\n", *entry.LogIndex)
+		fmt.Fprintf(os.Stderr, "Verified against tlog entry %d\n", *entry.LogIndex)
 		return cert, nil
 	}