Update readme a little bit (#17)

Author: ktoso

README.md

@@ -4,9 +4,73 @@
 
 Swift Distributed Tracing Extras ships the following extra modules:
 
-- [OpenTelemetrySemanticConventions](Sources/OpenTelemetrySemanticConventions)
+- [OpenTelemetrySemanticConventions](Sources/TracingOpenTelemetrySemanticConventions)
 
 ### 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).
+
+Semantic attributes enable users of [swift-distributed-tracing](https://github.com/apple/swift-distributed-tracing) to
+set attributes on spans using a type-safe pre-defined and well known attributes, like this:
+
+```swift
+import Tracing
+import TracingOpenTelemetrySemanticConventions
+
+// Example framework code, which handles an incoming request and starts a span for handling a request:
+func wrapHandleRequest(exampleRequest: ExampleHTTPRequest) async throws -> ExampleHTTPResponse {
+    try await InstrumentationSystem.tracer.withSpan(named: "test") { span in
+
+        // Set semantic HTTP attributes before
+        span.attributes.http.method = exampleRequest.method
+        span.attributes.http.url = exampleRequest.url
+        // ...
+
+        // Hand off to user code to handle the request;
+        // The current span already has all important semantic attributes set.
+        let response = try await actualHandleRequest(exampleRequest: exampleRequest)
+
+        // ... set any response attributes ... 
+        
+        return response
+    }
+}
+```
+
+You can also read attributes that are set on a span using the same property syntax:
+
+```swift
+assert(span.attributes.http.method == "GET")
+// etc.
+```
+
+Without the semantic attributes package, one would be able to set the attributes using a type-unsafe approach, like this:
+
+```swift
+span.attributes["http.method"] = "\(exampleRequest.method)"
+span.attributes["http.url"] = "\(exampleRequest.url)"
+```
+
+however this approach is error-prone as one can accidentally record not quite the right type of value,
+or accidentally use a wrong, unexpected, key that would then not be understood by Otel tracing backends.
+
+## Supported versions
+
+This module supports: **Swift 5.4+**, on all platforms Swift is available.