docs: adding KLT runtime info for local development (#1246)

Signed-off-by: Giovanni Liva <giovanni.liva@dynatrace.com>
Co-authored-by: Florian Bacher <florian.bacher@dynatrace.com>

functions-runtime/README.md

@@ -8,6 +8,30 @@ docker build -t keptnsandbox/klc-runtime:${VERSION} .
 
 ## Usage
 
+The Keptn function runtime uses [Deno](https://deno.land/)
+to execute Javascript/Typescript code.
+The Keptn Lifecycle Toolkit uses this runtime to run [KeptnTask](https://lifecycle.keptn.sh/docs/tasks/write-tasks/)
+for pre- and post-checks.
+The Keptn Lifecycle Toolkit passes parameters of `KeptnTask`s and
+[Context](https://lifecycle.keptn.sh/docs/concepts/tasks/#context) information
+to the runtime via the special environmental variable `DATA`.
+It also supports mounting Kubernetes secrets making them accessible via the `SECURE_DATA` env var.
+You can then read the data with the following snippet of code.
+
+```js
+const data = Deno.env.get("DATA")!;
+const secret = Deno.env.get("SECURE_DATA")!;
+console.log(data);
+console.log(secret);
+```
+
+`KeptnTask`s can be tested locally with the runtime using the following command.
+Replace `${VERSION}` with the KLT version of your choice.
+
+```sh
+docker run -v $(pwd)/test.ts:/test.ts -e SCRIPT=/test.ts -e DATA='{ "url":"http://localhost:9090" }' -e SECURE_DATA='{ "token": "myToken"}' -it ghcr.io/keptn/functions-runtime:${VERSION}
+```
+
 ### Docker with function on webserver (function in this repo)
 
 ```shell