Docs for breaking changes/new docs

[hzoo]

There's a lot of changes, too many to even think about so we should document more of them as we find/get reports.

• I think we need a new doc on config lookup Allow more flexible file-based configuration while preventing .babelrcs from breaking things babel#7358
• another one about how caching works: Cache plugins and presets based on their identity babel#6350
• updates to config files themselves (what kinds of files, what are the options, example configs) in the babelrc doc
• explain merging and "env" Merge all config & programmatic plugins/preset rather than duplicating babel#6905
• @babel/template examples, docs

[hzoo]

Ok closing in favor of #1628