Update changelog for w3c release.
### New Features
-+ Distributed Tracing now supports W3C Trace Context headers. When distributed
- tracing is enabled with `Config.DistributedTracer.Enabled = true`, the agent
- will now accept W3C's `traceparent` and `tracestate` headers when calling
- [`Transaction.AcceptDistributedTraceHeaders`](https://godoc.org/github.com/newrelic/go-agent/v3/newrelic#Transaction.AcceptDistributedTraceHeaders).
- When calling
- [`Transaction.InsertDistributedTraceHeaders`](https://godoc.org/github.com/newrelic/go-agent/v3/newrelic#Transaction.InsertDistributedTraceHeaders)
- the agent will include the W3C headers along with the New Relic distributed
- tracing header. To disable insertion of the New Relic distributed trace
- header (while keeping the W3C headers) use the
- `Config.DistributedTracer.ExcludeNewRelicHeader` configuration field.
+* Support for W3C Trace Context, with easy upgrade from New Relic trace context
+ * Distributed Tracing now supports W3C Trace Context headers for HTTP and
+ gRPC protocols when distributed tracing is enabled. Our implementation can
+ accept and emit both W3C trace header format and New Relic trace header
+ format. This simplifies agent upgrades, allowing trace context to be
+ propagated between services with older and newer releases of New Relic
+ agents. W3C trace header format will always be accepted and emitted. New
+ Relic trace header format will be accepted, and you can optionally disable
+ emission of the New Relic trace header format.
+ * When distributed tracing is enabled with
+ `Config.DistributedTracer.Enabled = true`, the Go agent will now accept
+ W3C's `traceparent` and `tracestate` headers when calling
+ [`Transaction.AcceptDistributedTraceHeaders`](https://godoc.org/github.com/newrelic/go-agent/v3/newrelic#Transaction.AcceptDistributedTraceHeaders). When calling
+ [`Transaction.InsertDistributedTraceHeaders`](https://godoc.org/github.com/newrelic/go-agent/v3/newrelic#Transaction.InsertDistributedTraceHeaders), the Go agent will include the
+ W3C headers along with the New Relic distributed tracing header, unless
+ the New Relic trace header format is disabled using
+ `Config.DistributedTracer.ExcludeNewRelicHeader = true`.
* Event data is now sent to New Relic every five seconds, instead of every
minute. As a result, transaction, custom, and error events will appear in
