Explicitly note JSON Lines output format

[alexchandel]

Related: #133

[alexchandel]

Is this good to merge?

[kristian-clausal]

At first I thought we were breaking some kind of requirement by not naming the files with .jsonl, but turns out that's just a "suggestion"; the three requirements are utf-8

• [x] utf-8
• [x] each line is valid JSON
• [x] line separator is '\n'

I don't see why not this small clarification couldn't be made.

[1over137]

I suggest you reconsider this. While it doesn't break a requirement to name it as .jsonl, it does break the expectation that a file named x.json is a valid json file. These are not valid json files, and will produce an error in any parser. Using .jsonl hints users and programs that this is something other than a json file.