Initial readme update

Author: ktoso

README.md

@@ -8,5 +8,21 @@ Swift Distributed Tracing Extras ships the following extra modules:
 
 ### OpenTelemetry Semantic Conventions
 
+#### Getting Started
+
+This module is complementary to [swift-distributed-tracing](https://github.com/apple/swift-distributed-tracing) so you will want to depend on it (the API package). 
+
+
+Then, you can depend on the extras package:
+
+```swift
+.package(url: "https://github.com/apple/swift-distributed-tracing.git", from: "..."),
+.package(url: "https://github.com/apple/swift-distributed-tracing-extras.git", from: "..."),
+```
+
+> If you are writing an application, rather than a library, you'll also want to depend on a specific tracer implementation. For available implementations refer to the [swift-distributed-tracing README.md](https://github.com/apple/swift-distributed-tracing).
+
+#### Usage
+
 The OpenTelemetry Semantic Conventions package provides span attributes to work with [OpenTelemetry's
 semantic conventions](https://github.com/open-telemetry/opentelemetry-specification/tree/main/specification/trace/semantic_conventions).