Adds link to json-schema-to-elm in implementations.md

[dragonwasrobot]

Adds a link to the json-schema-to-elm project in implementations.md.

[Relequestual]

It would be great if your project readme could specify which versions of JSON Schema it supports. Then I'll feel more able to approve and merge your request.

Thanks for your hard work =]

[Anthropic]

@dragonwasrobot I tried to check version for you on your project but as far as I can tell it supports draft 4 so far, is that correct? If you can add a *supports Draft x* to the end of the line or at least confirm the version here so I can add it for you that'd be great, sorry no one noticed this PR for so long! I should mention that drafts prior to 6 will be moved to the obsolete-implementations file, so the more recent the draft support the better.

[dragonwasrobot]

Hi, it currently only supports draft v7 (to avoid handling backwards-compatibility issues). I've made this more clear in the README of json-schema-to-elm and added to the description in the pull request.