Re-add documentation for http_json callback

The documentation for the http_json callback has been dropped
during a rebase from this review:

https://review.opendev.org/c/openstack/validations-common/+/723586

Change-Id: Ic5ac6f9aaf841dcc033234c3cbf928208257296d

README.rst

@@ -78,3 +78,31 @@ capable of running on most setups. But they are not meant for production environ
 
 .. _virtualenvwrapper: https://pypi.org/project/virtualenvwrapper/
 .. _Apache_license: http://www.apache.org/licenses/LICENSE-2.0
+
+
+Validations Callbacks
+=====================
+http_json callback
+------------------
+
+The callback `http_json` sends Validations logs and information to an HTTP
+server as a JSON format in order to get caught and analysed with external
+tools for log parsing (as Fluentd or others).
+
+This callback inherits from `validation_json` the format of the logging
+remains the same as the other logger that the Validation Framework is using
+by default.
+
+To enable this callback, you need to add it to the callback whitelist.
+Then you need to export your http server url and port::
+
+    export HTTP_JSON_SERVER=http://localhost
+    export HTTP_JSON_PORT=8989
+
+The callback will post JSON log to the URL provided.
+This repository has a simple HTTP server for testing purpose under::
+
+    tools/http_server.py
+
+The default host and port are localhost and 8989, feel free to adjust those
+values to your needs.