prepare docs for release 1.3.0

src/main/resources/gui/docs/api.md

@@ -1 +1,41 @@
-Put content here ;)
+The validation service listens to `POST`-requests to any server uri. You need to supply the xml/object to validate in the post body. 
+The service expects a single plain input in the post body, e.g. `multipart/form-data` is not supported.
+
+Examples:
+
+* `cURL`
+```shell script
+curl --location --request POST 'http://localhost:8080' \
+--header 'Content-Type: application/xml' \
+--data-binary '@/target.xml'
+```
+
+* `java` (Apache HttpClient)
+```java
+HttpClient httpClient = HttpClientBuilder.create().build();
+HttpPost postRequest = new HttpPost("http://localhost:8080/");
+FileEntity entity = new FileEntity(Paths.get("some.xml").toFile(), ContentType.APPLICATION_XML);
+postRequest.setEntity(entity);
+HttpResponse response = httpClient.execute(postRequest);
+System.out.println(IOUtils.toString(response.getEntity().getContent()));
+```
+
+* `javascript`
+```javascript
+var myHeaders = new Headers();
+myHeaders.append("Content-Type", "application/xml");
+
+var file = "<file contents here>";
+
+var requestOptions = {
+  method: 'POST',
+  headers: myHeaders,
+  body: file,
+  redirect: 'follow'
+};
+
+fetch("http://localhost:8080", requestOptions)
+  .then(response => response.text())
+  .then(result => console.log(result))
+  .catch(error => console.log('error', error));
+```