Adds upstream migration guide (#51)

docs/migration.md

@@ -0,0 +1,34 @@
+# Migrating to OpenTelemetry .NET
+
+All functionality provided by the distribution can also be utilized by manually
+installing and configuring upstream OpenTelemetry .NET packages. Follow these
+steps if you want to migrate from this distribution to the upstream
+OpenTelemetry .NET project.
+
+- Replace all environment variables or web.config/app.config values with the
+"grafana" prefix as explained [here](https://grafana.com/docs/grafana-cloud/send-data/otlp/send-data-otlp/#push-directly-from-applications-using-the-opentelemetry-sdks)
+- Install and activate the OpenTelemetry SDK,
+as covered in the upstream [Getting Started](https://github.com/open-telemetry/opentelemetry-dotnet#getting-started)
+guide
+- Setup the OTLP exporter for metrics, logs, and traces - see the
+[exporter README](https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/README.md)
+for details. The exporter will respect the previously set environment variables:
+  - `OTEL_EXPORTER_OTLP_PROTOCOL`
+  - `OTEL_EXPORTER_OTLP_ENDPOINT`
+  - `OTEL_EXPORTER_OTLP_HEADERS`
+- Install and configure any desired instrumentation packages
+[listed here](./supported-instrumentations.md)
+- Add `OTEL_SEMCONV_STABILITY_OPT_IN` environment variable to opt-in to the
+latest HTTP semantic conventions
+- [Add the recommended OpenTelemetry resource attributes](https://grafana.com/docs/opentelemetry/instrumentation/configuration/resource-attributes/)
+via the `OTEL_RESOURCE_ATTRIBUTES` environment variable
+  - `service.name`
+  - `service.namespace`
+  - `service.instance.id`
+  - `service.version`
+  - `deployment.environment`
+
+```shell
+export OTEL_SEMCONV_STABILITY_OPT_IN=http
+export OTEL_RESOURCE_ATTRIBUTES=service.instance.id=<pod123>,deployment.environment=...
+```