Definition of "breaking change"

A while ago this project adopted Semantic Versioning (#9) and so far there are two releases out following that specification [0.1.0](https://github.com/greenled/portainer-stack-utils/releases/tag/0.1.0) and [0.1.1](https://github.com/greenled/portainer-stack-utils/releases/tag/0.1.1). As new ones are on their way, there is a need for a clear definition of what is considered a "breaking change" and what is not.

[From the spec](https://semver.org/#spec-item-1):

> Software using Semantic Versioning MUST declare a public API. This API could be declared in the code itself or exist strictly in documentation. However it is done, it SHOULD be precise and comprehensive.

So, what/where is this project's public API to start with?:

- Executable's input/output format?
- Docker image definition?
- Settings?
- Algorithms (and source code in general)?
- Supported platforms?
- External dependencies?
- A mix of the above?

This issue is for addresing those questions. Findings should be added to the Readme.