add readme

Signed-off-by: bharath-techie <[email protected]>

Author: bharath-techie

plugins/engine-datafusion/README.md

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+
+## Prerequisites
+
+1. Publish OpenSearch to maven local
+```
+./gradlew publishToMavenLocal
+```
+2. Publish SQL plugin to maven local
+```
+./gradlew publishToMavenLocal
+```
+3. Run opensearch with following parameters
+```
+ ./gradlew run --preserve-data -PremotePlugins="['org.opensearch.plugin:opensearch-job-scheduler:3.3.0.0-SNAPSHOT', 'org.opensearch.plugin:opensearch-sql-plugin:3.3.0.0-SNAPSHOT']" -PinstalledPlugins="['engine-datafusion']" --debug-jvm
+```
+
+
+## Steps to test indexing + search e2e
+
+TODO : need to remove hardcoded index name `index-7`
+
+1. Delete previous index if any
+```
+curl --location --request DELETE 'localhost:9200/index-7'
+```
+
+2. Create index with name : `index-7`
+```
+curl --location --request PUT 'http://localhost:9200/index-7' \
+--header 'Content-Type: application/json' \
+--data-raw '{
+    "settings": {
+        "number_of_shards": 1,
+        "number_of_replicas": 0,
+        "refresh_interval": -1
+    },
+    "mappings": {
+        "properties": {
+            "message": {
+                "type": "long"
+            },
+            "message2": {
+                "type": "long"
+            },
+            "message3": {
+                "type": "long"
+            }
+        }
+    }
+}'
+```
+3. Index docs
+```
+curl --location --request POST 'http://localhost:9200/_bulk' \
+--header 'Content-Type: application/json' \
+--data-raw '{"index":{"_index":"index-7"}}
+{"message": 2,"message2": 3,"message3": 4}
+{"index":{"_index":"index-7"}}
+{"message": 3,"message2": 4,"message3": 5}
+'
+```
+4. Refresh the index
+```
+curl localhost:9200/index-7/_refresh
+```
+5. Query
+```
+curl --location --request POST 'http://localhost:9200/_plugins/_ppl' \
+--header 'Content-Type: application/json' \
+--data-raw '{
+  "query": "source=index-7 | stats count(), min(message) as min, max(message2) as max"
+}'
+```