Better documentation of elm-format's design philosophy

[avh4]

A brief mention of some high-level goals are mentioned in the README, but there's a lot more undocumented about the philosophy. For this this issue will be a place for me to dump snippets that hopefully can be organized later.

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Regarding "lenient parser" features: I primarily want to design for automatically fixing mistakes; rather than making convoluted rules that are shortcuts for advanced users ... I think if it's valid Elm, then clearly it should parse the same as Elm compiler, and if it's not, there'd need to be a pretty simple set of rules that would define what happens