[MSHADE-381] Introduction of YamlAppendingTransformer

**[Peter Butkovic](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=typek_pb)** opened **[MSHADE-381](https://issues.apache.org/jira/browse/MSHADE-381?redirect=false)** and commented

As documented in the official docs (https://maven.apache.org/plugins/maven-shade-plugin/examples/resource-transformers.html#AppendingTransformer ), there are following specific types of Appending Transformers: XmlAppendingTransformer and ResourceBundleAppendingTransformer.

I'd however need YamlAppendingTransformer capable of merging:

1.yaml

```java
key1:
  propA: valA
```

2.yaml

```java
key1:   
propB: valB
```

into:

```java
key1:  
propA: valA
propB: valB
```

---

**Affects:** 3.2.4

The issue names the resource-transformers documentation but no source file, test, or entry point. Start with that documentation and the two YAML examples; done means a YamlAppendingTransformer merges them into the shown combined structure.

Written by the indexing model from the issue text.