Update readme with instructions for sending data to local ES instance (#169)

Author: sorenlouv

.github/README.md

@@ -30,6 +30,25 @@ Additionally, the OpenTelemetry Contrib collector has also been changed to the [
 3. Click "Create an API Key" to create one.
 4. Run `./demo.sh serverless docker`
 
+#### Connect to a local Elasticsearch cluster
+The following steps shows how to start the Otel demo in a Docker container and send the generated otel data to an Elasticsearch instance running locally on the host.
+
+1. Create an API key
+```sh
+curl -X POST "http://localhost:9200/_security/api_key" -u USER:PASSWORD -H "Content-Type: application/json" -d'{ "name": "my_api_key" }'
+```
+
+2. Update `.env.overide` with URL and API key:
+```yml
+ELASTICSEARCH_ENDPOINT="http://host.docker.internal:9200"
+ELASTICSEARCH_API_KEY="<api key obtained in step 2>"
+```
+3. Start the Otel demo in a Docker container:
+
+```sh
+make start
+```
+
 
 ### Manual Installation
 <details> 
@@ -149,4 +168,4 @@ Suppose you want to see how your new processor is going to play out in this demo
 ## Clean-up 
 
 - **Docker**. Run `./demo.sh destroy docker`
-- **Kubernetes**. Run `./demo.sh destroy k8s`
+- **Kubernetes**. Run `./demo.sh destroy k8s`