Add more information about contributions to the README

Author: m.hollink

README.md

@@ -46,3 +46,31 @@ We welcome any and all contributions! Here are some ways you can get started:
 4. Commit and push
 5. Submit a pull request
 6. Wait for pull request to be merged
+
+## Running the project locally
+
+The application is build up into 3 main parts; The webapp, an API and a database. Each of these component can be run
+locally which allows you to make some changes and test these before commiting / submitting a pull request.
+
+### The API & Database
+
+The API and database can be started locally using [Docker](https://www.docker.com/). There is a Dockerfile inside
+the /api folder which can be used to create an image containing the required environment to run the Slim4 API. Since the
+API is connecting to a database and a connection is required between its easier to spin everything up using
+docker compose.
+
+```shell
+# from the root directory:
+docker compose up
+```
+
+> Note: Running the api container automatically creates/overwrites your .env file with the local database credentials.
+
+#### API Client
+
+After the containers are started you can open up an API client like [Postman](https://www.postman.com/)
+or [Thunder Client](https://www.thunderclient.com/). There is a postman collection in the repository you can
+use if you decide on using Postman.
+
+> Note: Each request must have an Authorization header consisting a bearer token. For development, it's not necessary to
+> have a valid token (as its being mocked in the [$tokenMiddleware](./api/src/middleware/token-verification.php)).