[MSHADE-266] Make more clear that ManifestResourceTransformer can add *any* entry

**[Bradley Turek](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=turekbot)** opened **[MSHADE-266](https://issues.apache.org/jira/browse/MSHADE-266?redirect=false)** and commented

I suggest that it be made blatantly clear that the ManifestResourceTransformer can add any name-value pair.

Maybe by example. What if we add the following?

> ## Setting Manifest Entries with the ManifestResourceTransformer
>
> The ManifestResourceTransformer allows existing entries in the MANIFEST to be replaced and new entries added.
>
> Manifest entries are added using the following format:
> `Value`
>
> For example, having a configuration of
>
> ```xml
> "org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
>
> com.foo.Main
> 1.5.0
> arbitrary value
>
>
> ```
>
> will result in a `MANIFEST.MF` of
>
> ```
> ...
> Main-Class: com.foo.Main
> Implementation-Version: 1.5.0
> Arbitrary-Entry: arbitrary value
> ...
> ```
>
>
> Here's a full example configuration with, the following sample entries and values
>
> the Main-Class entry to the value of the app.main.class property,
> the X-Compile-Source-JDK entry to the value of the maven.compile.source property and
> the X-Compile-Target-JDK entry to the value of the maven.compile.target property.
> ...

---

**Affects:** 3.1.0

Start by locating the documentation for ManifestResourceTransformer and its manifestEntries configuration. Update the explanation with an example showing that arbitrary name-value entries can be added, then verify that the rendered documentation clearly reflects the requested MANIFEST.MF output.

Written by the indexing model from the issue text.