add directory for common assets

common/protos/meshtastic/rtttl.proto

@@ -0,0 +1,19 @@
+syntax = "proto3";
+
+package meshtastic;
+
+option csharp_namespace = "Meshtastic.Protobufs";
+option go_package = "github.com/meshtastic/go/generated";
+option java_outer_classname = "RTTTLConfigProtos";
+option java_package = "com.geeksville.mesh";
+option swift_prefix = "";
+
+/*
+ * Canned message module configuration.
+ */
+message RTTTLConfig {
+  /*
+   * Ringtone for PWM Buzzer in RTTTL Format.
+   */
+  string ringtone = 1;
+}