docs: improve gruf instrumentation documentation

instrumentation/gruf/README.md

@@ -1,6 +1,19 @@
 # OpenTelemetry Gruf Instrumentation
 
-Todo: Add a description.
+The OpenTelemetry Gruf Ruby gem is a community-maintained instrumentation for Gruf, a gRPC framework for Ruby. It enables automatic tracing of RPC requests handled by Gruf services.
+
+## Overview
+
+This instrumentation integrates OpenTelemetry with Gruf to create spans for incoming gRPC requests. It helps in observing request flow, latency and errors in distributed systems.
+
+## How it works
+
+The Gruf instrumentation hooks into the request lifecycle of Gruf-based gRPC services and automatically creates spans for each incoming RPC request.
+
+It captures useful metadata such as:
+- RPC method name
+- Request lifecycle events
+- Errors, if any
 
 ## How do I get started?
 
@@ -35,6 +48,12 @@ OpenTelemetry::SDK.configure do |c|
 end
 ```
 
+## Key Files
+
+- `instrumentation.rb` — Entry point for enabling the instrumentation
+- `middleware/` — Contains logic for intercepting and tracing requests
+- `version.rb` — Defines the gem version
+
 ## Examples
 
 Example usage can be seen in the [`./example/trace_demonstration.rb` file](https://github.com/open-telemetry/opentelemetry-ruby-contrib/blob/main/instrumentation/gruf/example/trace_demonstration.rb)