Document the necessity of fixities.

fourmolu.yaml

@@ -9,6 +9,21 @@ newlines-between-decls: 1 # number of newlines between top-level declarations
 single-constraint-parens: auto
 let-style: inline
 
+# The fixity of operators affects how Fourmolu formats them.
+# Without this directive, we'd get really wonky corrections that would do things like turn this:
+# ```
+# exprA && exprB
+# ```
+# Into
+# ```
+# expr
+#  && exprB
+# ``
+# For operator heavy code, like Aeson parsers, it would be even worse than this.
+# Fourmolu says that it can automatically detect fixities in most cases,
+# but I had to specify these manually to get reasonable formatting.
+# Docs: https://github.com/fourmolu/fourmolu#language-extensions-dependencies-and-fixities
+# If you have issues with operator formatting, usually you can find the right fixity in the haddocks.
 fixities:
     - infixl 3 <|>
     - infix 4 ==