Find projects that use the maven plugin

[nedtwigg]

We are able to get a rough count of our Gradle users using https://github.com/search?l=gradle&q=spotless&type=Code
That's nice for helping users trust our plugin, and also to find how other people are using it.

If you filter that search by xml rather than Gradle https://github.com/search?l=XML&q=spotless&type=Code You get some pom.xml, but a lot of other random stuff too.

If we make a toy pom.xml, we should be able to find every maven project on GitHub using our plugin, using https://help.github.com/en/github/visualizing-repository-data-with-graphs/listing-the-projects-that-depend-on-a-repository

[nedtwigg]

GitHub search now supports limiting the search to Maven POM, super helpful!

• Minor tweaks to homepage READMEs #1534