Add section on using a baseline

docs/dealing_with_code_issues.md

@@ -74,3 +74,19 @@ function (int $a) : string {
   return $a;
 }
 ```
+
+## Using a baseline file
+
+If you have a bunch of errors and you don't want to fix them all at once, Psalm can now grandfather-in errors in existing code, while ensuring that new code doesn't have those same sorts of errors.
+
+```
+vendor/bin/psalm --set-baseline=your-baseline.xml
+```
+
+will generate a file containing the current errors. You can commit that generated file so that Psalm running in other places (e.g. CI) won't complain about those errors either, and you can update that baseline file (to remove references to things that have been fixed) with
+
+```
+vendor/bin/psalm --update-baseline
+```
+
+Your mileage may vary, but we've found baseline files to be a great way to gradually improve a codebase.