docs: add instructions to get transaction id

Author: mamoonraja

README.md

@@ -267,6 +267,26 @@ Headers: "<http response headers>"
 Result: "<response returned by service>"
 ```
 
+### Transaction IDs
+Every SDK call will return a response which will contain a transaction ID, accessible via the `x-global-transaction-id` header. This transaction ID is useful for troubleshooting and accessing relevant logs from your service instance.
+
+
+```ruby
+require "ibm_watson"
+include IBMWatson
+
+assistant = AssistantV1.new(
+  authenticator: "<authenticator>"
+  version: "2017-04-21"
+)
+
+response = assistant.headers(
+  "Custom-Header" => "custom_value"
+  ).list_workspaces
+
+p "Global Transaction Id: #{response.headers["X-Global-Transaction-Id"]}"
+```
+
 ## Configuring the HTTP client
 
 To set client configs like timeout or proxy use the `configure_http_client` function and pass in the configurations.
@@ -399,4 +419,4 @@ We'd love to highlight cool open-source projects that use this SDK! If you'd lik
 [license]: http://www.apache.org/licenses/LICENSE-2.0
 [vcap_services]: https://cloud.ibm.com/docs/services/watson?topic=watson-vcapServices
 [ibm-cloud-onboarding]: http://cloud.ibm.com/registration?target=/developer/watson&cm_sp=WatsonPlatform-WatsonServices-_-OnPageNavLink-IBMWatson_SDKs-_-Ruby
-[ivar]: http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/IVar.html
+[ivar]: http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/IVar.html