Add credential-free sample mode with sample data, README, and doc reconciliation

internal/geocode/fake.go

@@ -0,0 +1,13 @@
+package geocode
+
+import "crypto/sha256"
+
+type Fake struct{}
+
+func (Fake) Lookup(address string) (Point, error) {
+	sum := sha256.Sum256([]byte(address))
+	return Point{
+		Lat: 37.5 + (float64(sum[0])/255-0.5)*0.12,
+		Lng: -122.45 + (float64(sum[1])/255-0.5)*0.12,
+	}, nil
+}