add documentation on how to test the processor

Author: multicatch

README.md

@@ -28,3 +28,30 @@ In short, the steps to run your custom processor are the following:
 3. Prepare `processor.properties` with rdkafka (Kafka client) configuration - [see example](./processor.properties) (put this file in the same directory as your executable).
 4. Run your executable (to see logs set the following environment variable: `RUST_LOG=info`, e.g. in bash you can just run `RUST_LOG=info ./your_executable`).
 
+## Test your processor
+
+You may want to test if the generated processor is correct before deploying it.
+To test it, in the generated project, create a `simulations` directory. 
+In this directory, create another directory (or directories, depending on the `template.yaml`) with the name `${input_topic}_${output_topic}`,
+where `${input_topic}` is the name of the input_topic from `template.yaml` and `${output_topic}` is the name of the output_topic from `template.yaml`.
+
+For example, if you have the following stream in your `tempate.yaml`:
+```yaml
+streams:
+  - input_topic: sometopic
+    output_topic: target
+```
+
+Then create the following directory structure:
+```text
+<project_directory>
+ > simulations
+ | > sometopic_target
+```
+
+In the `${input_topic}_${output_topic}` directory (in this case - `sometopic_target`), create text files with the input message and expected output.
+For example, given the template [`all_processors.yaml`](template-examples/all_processors.yaml) (see template-examples), I have prepared some test data in [`simulations/in_out`](simulations/in_out).
+The test files always have two headers - `[Input]` (for input JSON) and `[Expected]` (for expected processed message).
+
+To run the simulation, run `cargo test` in the generated project. 
+See [`kjp-generator/tests/integration_test.rs`](kjp-generator/tests/integration_test.rs) for a complete example.