Move documentation to a dedicated directory.

docs/file-pattern-matchers.md

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+This is how we convert a file pattern string found in a .editorconfig to a Go-compatible regex. This
+is done mostly according to the rules documented under "Wildcard Patterns" here:
+http://docs.editorconfig.org/en/master/editorconfig-format.html#patterns
+
+The are some differences to the official documentation, however.
+
+`*` Matches any string of characters, except path separators (/).
+
+`**` Matches any string of characters.
+
+`?` Matches any single character.
+
+`[seq]` Matches any single character in seq.
+
+`[!seq]` Matches any single character not in seq.
+
+`{s1,s2,s3}` Matches any of the strings given (separated by commas, can be nested).
+
+`{num1..num2}` Matches any integer numbers between num1 and num2, where num1 and num2 can be either positive or negative.
+
+**Differences from the official document**
+
+If file pattern is entirely `*`, according to the official documentation it should match files in the
+same directory and not in sub-directories (because `*` excludes path separators). However, in open
+source repositories I reviewed, a single `*` seems to be universally used to mean every file,
+instead of the technically-correct `**`. We adapt to what is used in the real world to be practical.