Add release notes for ODFE 1.7.0 (opendistro-for-elasticsearch#119)

* Add ODFE 1.7.0 release notes

release-notes/release-notes-odfe-1.7.0.md

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+## Open Distro for Elasticsearch 1.7.0 Release Notes
+Compatible with Elasticsearch 7.6.1 and Open Distro for Elasticsearch 1.7.0.
+
+## Initial Release
+The Anomaly Detection Elasticsearch plugin enables you to detect anomalies in streaming time series data based on the random cut forest (RCF) algorithm.
+You can create anomaly detectors and add features to them to customize what data you want to detect anomalies over.
+
+This plugin can be used with the [Anomaly Detection Kibana plugin](https://github.com/opendistro-for-elasticsearch/anomaly-detection-kibana-plugin) for an intuitive user interface that can be used to configure, start, and stop anomaly detectors. You can also view the anomaly history for all of your created detectors. 
+
+You can use the plugin with the same version of the [Open Distro Alerting plugin](https://github.com/opendistro-for-elasticsearch/alerting) and [Open Distro Alerting Kibana plugin](https://github.com/opendistro-for-elasticsearch/alerting-kibana-plugin) to get alert notifications. You can create a monitor based on an anomaly detector directly on the Alerting Kibana plugin. Monitors run checks on the anomaly detection results regularly and trigger alerts based on custom trigger conditions.
+
+## Features
+
+1. Create and configure anomaly detectors over user-specified indices and features
+2. Start and stop detectors at any time
+3. Query anomaly results
+4. Query existing detectors
+5. Query specific detector details and current state
+6. Preview anomaly results for new detector features
+
+## Current Limitations
+
+- Limit of 1000 detectors per Elasticsearch cluster
+- Limit of 5 features per detector
+- Total detectors memory limit of 10% of JVM heap
+- Not all API calls have complete error handling
+- We will continuously add new unit test cases, but we don't have 100% unit test coverage for now. This is a great area for developers from the community to contribute and help improve test coverage
+- Please see documentation links and GitHub issues for other details