added readme for prometheus loader

llama_hub/prometheus/README.md

@@ -0,0 +1,31 @@
+# Prometheus Loader
+
+This loader loads metrics from Prometheus. The user specifies a Prometheus instance to initialize the reader. They then specify the query, the date range, steps, Http parameters and which fields must be used as metadata.
+
+Automatically adds to the metadata the timestamp value and create a document per value returned by the prometheus query.
+
+## Usage
+
+Here's an example usage of the PrometheusReader.
+
+```python
+from llama_index import download_loader
+import os
+
+PrometheusReader = download_loader('PrometheusReader')
+
+endpoint = "<endpoint>"
+size = "<size>"
+# query_dict is passed into db.collection.find()
+query = ""
+end_time = datetime.utcnow()
+start_time = end_time - timedelta(hours=1)
+metadata_fields = ['id', "host"]
+reader = PrometheusReader(endpoint, size)
+documents = reader.load_data(query=query, 
+                             start_time=start_time,
+                             end_time=end_time,
+                             step="1m",
+                             get_pararameters=None,
+                             metadata_fields=metadata_fields)
+```