docs(otlp-transformer): add entrypoints to readme.md

Author: pichlermarc

experimental/packages/otlp-transformer/README.md

@@ -19,6 +19,22 @@ To get started you will need to install a compatible OpenTelemetry API.
 npm install @opentelemetry/api
 ```
 
+### Entrypoints
+
+Since this package depends on all signals, and protobuf.js for serialization,
+this package offers multiple entrypoints to avoid polluting the app during bundling.
+
+- `@opentelmetry/otlp-transformer` interfaces used by every signal
+- `@opentelmetry/otlp-transformer/metrics` interfaces used by metrics
+- `@opentelmetry/otlp-transformer/metrics/protobuf` OTLP protobuf serialization for metrics
+- `@opentelmetry/otlp-transformer/metrics/json` OTLP JSON serialization for metrics
+- `@opentelmetry/otlp-transformer/trace` interfaces used by trace
+- `@opentelmetry/otlp-transformer/trace/protobuf` OTLP protobuf serialization for trace
+- `@opentelmetry/otlp-transformer/trace/json` OTLP JSON serialization for trace
+- `@opentelmetry/otlp-transformer/experimental/logs` interfaces used by logs
+- `@opentelmetry/otlp-transformer/experimental/logs/protobuf` OTLP protobuf serialization for logs
+- `@opentelmetry/otlp-transformer/experimental/logs/json` OTLP JSON serialization for logs
+
 ### Serialize Traces/Metrics/Logs
 
 This module exports serializers to serialize traces, metrics and logs from the OpenTelemetry SDK into protocol buffers