docs: show coverage in README

Adds the coveralls badge in the README to showcase immediately the backend
coverage.

Signed-off-by: Luca Zaninotto <luca.zaninotto@secomind.com>

Author: lusergit

README.md

@@ -1,11 +1,26 @@
-<!---
-  Copyright 2021,2022 SECO Mind Srl
+<!--
+This file is part of Edgehog.
 
-  SPDX-License-Identifier: Apache-2.0
+Copyright 2021, 2022, 2026 SECO Mind Srl
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+SPDX-License-Identifier: Apache-2.0
 -->
 
 ![Backend CI](https://github.com/edgehog-device-manager/edgehog/actions/workflows/backend-ci.yaml/badge.svg)
 ![Frontend CI](https://github.com/edgehog-device-manager/edgehog/actions/workflows/frontend-ci.yaml/badge.svg)
+![Coverage](https://img.shields.io/coverallsCoverage/github/edgehog-device-manager/edgehog)
 
 # Edgehog