[general] Improve docs, showcase and contributing section

[realvictorprm]

Currently the github page of Embeddinator is lacking detailed information about how it works, where it's useful, how to contribute etc. I consider this as very important.

My suggestions is following:
Start with adding a wiki which describes in more detail how to use it, add some examples both to the repository and the wiki and at least at a contributing.md .

[VincentDondain]

+1 for the wiki in general, as a way to give an overview of the whole project, explain the branches, link to builds and detail how to contribute.

Let us also keep in mind that we might want to avoid wiki pages for some files (like getting started in #223 for instance for instance) as we'll have to push them to our documentation website.

[chamons]

Ok #646 has starting versions of everything mentioned here except showcase / more samples. These should likely wait until we figure out the msbuild story, as that will change them pretty dramatically.