Suggested changes to the README.md

[tlimoncelli]

1. Reorg the list of providers

Now that the list is so long, we probably want to list them in a more compact way, and group them.

Some thoughts:

• Instead of 2 lists (DSPs and Registrars), one lists with an "*" to mean "registrar only".
• Instead of a long list, use be a table with 5 names across.
• Group them by category: "Cloud Providers", "DNS SaaS providers", "Self-Hosted" (BIND, PowerDNS), "Network Hardware"

2. The opening paragraph should use more updated terminology. IaC instead of DSL.

DNSControl is Infrastructure as Code for DNS. It includes a full-featured configuration language (Javascript-compatible), plus "plug-ins" that speak to DNS provider APIs. It can upload the same DNS records to multiple providers to improve reliability. It even generates the most beautiful BIND zone files ever. It runs anywhere Go runs (Linux, macOS, Windows, FreeBSD). It is distributed as source, binaries, and Docker containers. Get started easily with the import feature!

3. Move the example up higher

The example really sells it. Move that up.

[tlimoncelli]

CC @cafferata Thoughts?