add documentation

Author: gugahoa

processors/opentelemetry_baggage_processor/README.md

@@ -25,6 +25,44 @@ Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_do
 and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
 be found at [https://hexdocs.pm/opentelemetry_baggage_processor](https://hexdocs.pm/opentelemetry_baggage_processor).
 
-## Configuration
+## Usage
 
-TODO: Add configuration instructions
+<!-- MDOC -->
+
+`OpentelemetryBaggageProcessor` provides a [Span Processor](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#span-processor)
+that takes attributes from the [Baggage](https://hexdocs.pm/opentelemetry_api/otel_baggage.html)
+and insert into the Span, once it starts.
+
+A Span Processor is not an application, to use it you must update your
+configuration:
+
+```elixir
+# config/config.exs
+
+config :opentelemetry,
+  processors: [{:otel_baggage_processor, %{}}]
+```
+
+If you already have another processor defined, like `:otel_batch_processor`, add
+the tuple and don't override the whole configuration.
+
+Now every new span should have what's inside your baggage as attribute.
+
+### Options
+
+* `:prefix` - adds a prefix for all baggage attributes.
+* `:filter` - only add attributes if the baggage metadata has the configured key.
+The key must be a binary.
+
+### Limitations
+
+Baggage will follow the Context. So any limitation to Context Propagation applies
+to Baggage Propagation, and thus to what attributes are going to be added to your
+Span.
+
+We can only apply attributes on Span's start, since that's when we can modify
+them. There's a [BeforeEnd callback proposal](https://github.com/open-telemetry/opentelemetry-specification/issues/1089)
+which would allow us to add the Baggage's attribute on Span's end too, but that
+remains as something to be revisited in the future.
+
+<!-- MDOC -->