Add fixity troubleshooting in README

README.md

@@ -18,6 +18,8 @@
     * [Regions](#regions)
     * [Exit codes](#exit-codes)
     * [Using as a library](#using-as-a-library)
+* [Troubleshooting](#troubleshooting)
+    * [Operators are being formatted weirdly!](#operators-are-being-formatted-weirdly)
 * [Limitations](#limitations)
 * [Running on Hackage](#running-on-hackage)
 * [Forks and modifications](#forks-and-modifications)
@@ -272,6 +274,32 @@ For these purposes only the top `Ormolu` module should be considered stable.
 It follows [PVP](https://pvp.haskell.org/) starting from the version
 0.5.3.0. Rely on other modules at your own risk.
 
+## Troubleshooting
+
+### Operators are being formatted weirdly!
+
+This can happen when Ormolu doesn't know or can't determine the fixity of an
+operator.
+
+* If this is a custom operator, see the instructions in the [Language
+  extensions, dependencies, and
+  fixities](#language-extensions-dependencies-and-fixities) section to
+  specify the correct fixities in a `.ormolu` file.
+
+* If this is a third-party operator (e.g. from `base` or some other package
+  from Hackage), Ormolu probably doesn't recognize that the operator is the
+  same as the third-party one.
+
+  Some reasons this might be the case:
+
+    * You might have a custom Prelude that re-exports things from Prelude
+    * You might have `-XNoImplicitPrelude` turned on
+
+  If any of these are true, make sure to specify the reexports correctly in
+  a `.ormolu` file.
+
+You can see how Ormolu decides fixity of operators if you use `--debug`.
+
 ## Limitations
 
 * CPP support is experimental. CPP is virtually impossible to handle