Add an Implementations.md

This idea arose from the discussion in #159. In the root of the repository, I want to create a file named Implementations.md that lists all known benign Mustache implementations.

This will make it possible to list more than one implementation per host programming language and to briefly describe the differences between them. The mechanic of listing language support on the home page can stay the same, but the languages.yml can link to a section of the Implementations.md for those languages where there is more than one viable option, instead of linking directly to one of them.

In addition to mentioning languages directly on the home page, I want to add a link at the end of that listing that points to the whole Implementations.md. It should look something like "(see all)".