docs: add basic README

SVC-1398

README.md

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+# hale-transformer-api
+
+Spring Boot application that runs hale transformations based on requests read from a AMQP message queue.
+The message queue needs to be provided externally, e.g. by a RabbitMQ instance.
+
+## Build
+
+To build the application, run
+
+    ./gradlew build
+
+To start an instance, run
+
+    ./gradlew bootRun
+
+## RabbitMQ
+
+To start a local RabbitMQ instance that can be used for debugging purposes, run
+
+    docker compose up
+
+in the root directory of this project.
+
+The local instance can be also used to send messages to the queue that are then
+processed by the application. When the application is running, an example message
+like the following can be sent to the queue:
+
+```bash
+$ docker exec -ti hale-transformer-api-rabbitmq-1 bash
+root@123456789abc:/# rabbitmqadmin publish \
+> exchange=hale-transformer-exchange \
+> routing_key=hale.transformation.foo \
+> properties='{"content_type":"application/json"}' payload='{"projectUrl": "http://example.org/proj", "sourceDataUrl": "http://example.org/data"}'
+Message published
+root@123456789abc:/#
+```