Project conventions: documentation

One of our conventions is not yet documented and not enforced with `bin/check*` scripts.
It is:
- There should be comments explaining and linking to documentation
- Please compare: [google/google_compute_instance/simple/main.tf](https://github.com/ContainerSolutions/terraform-examples/blob/main/google/google_compute_instance/simple/main.tf)

The comments that we want to have in all examples:
- `# Summary:` – 1-line summary of what this example does
- `# Documentation:` – add link to documentation before each block (terraform, provider, variable, resource, etc.)
- `# Explanation:` – add only where some extra explanation is needed

TODO:

- [x] add this convention to README
- [ ] #183