Reset readme, now that docs will be on official website

[webmat]

This PR has been replaced by #400

In this PR:

• move generated README.md to generated/legacy/README.md
• point to the official docs on the main site
• guide people to various contributor-centric documentation
• document generated files
• align the generated/ecs/*.yml file names to the corresponding variables in the code generators
• rename schemas/format.md to schemas/README.md so it renders by default, when visiting directory on GitHub

TODO

• Replace links to main website:
  • Link to top of documentation section, in README.md
  • Link to "Guidelines and Best Practices", in CONTRIBUTING.md
• Actually, perhaps we should move the old generated README.md to generated/legacy for now

[webmat]

Reviewers can navigate this new readme and other files at https://github.com/webmat/ecs/tree/readme-redirect

[ruflin]

What is the reason you keep all the old files around? Can't we just delete them and in case we need them again we use git?

[ruflin]

I would not put changing docs into the README.

[webmat]

I agree in principle. However I thought since the official ECS reference used to be this file, it's disappearance was worth a mention in the changelog

[webmat]

Yeah I was on the fence on the old files. I'll delete them and all of the code associated.

[webmat]

Old readme is now deleted.

I'll get rid of the other generated files in another PR, since this PR is meant to be one of the concrete steps of the asciidoc deployment to the main site. I don't want it to contain too much.

[webmat]

This PR was getting a bit too big for my taste, for something that should be done on the day of the deployment to the main website.

I'm closing it in favour of the trimmed down #400