#1113 add README for the doc.

analyzers/OpenCTI/README.md

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+[OpenCTI](https://www.opencti.io/en/) is an open cyber threat intelligence platform which aims at providing a powerful knowledge management database with an enforced schema especially tailored for cyber threat intelligence and cyber operations and based on STIX 2.
+
+The analyzer comes in only one flavor to look for an observable in the platform.
+The analyzer comes in two flavors to search for an observable in the platform:
+
+- OpenCTI_**SearchExactObservable**: returns an exact match only
+- OpenCTI_**SearchObservables**: returns all observables containing the input data
+
+#### Requirements
+
+The OpenCTI analyzer requires you to have access to one or several [OpenCTI](https://www.opencti.io/en/)
+ instances. You can also deploy your own instance.
+ instances in version 4. You can also deploy your own instance.
+
+Three parameters are required for each instance to make the analyzer work:
+
+- `url` : URL of the instance, e.g. "https://demo.opencti.io"