PEP 518: Cite @njsmith's config file format review (#668)

* PEP 518: Cite @njsmith's config file format review

* Add a file format feature summary table

pep-0518.txt

@@ -255,6 +255,44 @@ vendored easily by projects. This outright excluded certain formats
 like XML which are not friendly towards human beings and were never
 seriously discussed.
 
+Overview of file formats considered
+'''''''''''''''''''''''''''''''''''
+
+The key reasons for rejecting the other alternatives considered are
+summarised in the following sections, while the full review (including
+positive arguments in favour of TOML) can be found at [#file_formats]_.
+
+TOML was ultimately selected as it provided all the features we
+were interested in, while avoiding the downsides introduced by
+the alternatives.
+
+======================= ==== ==== ==== =======
+Feature                 TOML YAML JSON CFG/INI
+======================= ==== ==== ==== =======
+Well-defined            yes  yes  yes
+Real data types         yes  yes  yes
+Reliable Unicode        yes  yes  yes
+Reliable comments       yes  yes
+Easy for humans to edit yes  ??        ??
+Easy for tools to edit  yes  ??   yes  ??
+In standard library               yes  yes
+Easy for pip to vendor  yes       n/a  n/a
+======================= ==== ==== ==== =======
+
+("??" in the table indicates items where most folks would be
+inclined to answer "yes", but there turn out to be a lot of
+quirks and edge cases that arise in practice due to either
+the lack of a clear specification, or else the underlying
+file format specification being surprisingly complicated)
+
+The ``pytoml`` TOML parser is ~300 lines of pure Python code,
+so being outside the standard library didn't count heavily
+against it.
+
+Python literals were also discussed as a potential format, but
+weren't considered in the file format review (since they're not
+a common pre-existing file format).
+
 
 JSON
 ''''
@@ -485,6 +523,9 @@ References
 .. [#jsonschema] JSON Schema
    (http://json-schema.org/)
    
+.. [#file_formats] Nathaniel J. Smith's file format review
+   (https://gist.github.com/njsmith/78f68204c5d969f8c8bc645ef77d4a8f)
+
 
 Copyright
 =========