Add a list of upcoming breaking changes

[orangejulius]

We try to avoid breaking changes in Pelias, but sometimes they are required.

While we have a roadmap document for new features, it might make sense to explicitly call out things that we expect or plan to build a breaking change around soon.

Some examples of what we could list currently:

• Dropping support for Node.js 8
• Defaulting imports.whosonfirst.sqlite to true
• Requiring a separate libpostal service for interpolation
• Dropping support for Elasticsearch 5