Add section to README about WP global constants

Global constants such as those for paths can't be stubbed because they need to be literal string paths to the actual locations of included/required files.

README.md

@@ -83,6 +83,30 @@ If a directory is provided, the plugin will search for the following files:
 
 The plugin expects a JSON representation of the hooks as per [wp-hooks/generator](https://github.com/wp-hooks/generator).
 
+### WordPress paths
+
+To help Psalm analyze your project you might need to define some of WordPress' default global constants such as those for paths.
+
+```xml
+<?xml version="1.0"?>
+<psalm autoloader="tests/bootstrap.php" xmlns="https://getpsalm.org/schema/config">
+	<!-- project configuration -->
+</psalm>
+```
+
+The following example bootstrap file is for a Bedrock installation:
+
+```php
+<?php
+
+require_once dirname( __DIR__ ) . '/config/application.php';
+
+define( 'WPMU_PLUGIN_DIR', WP_CONTENT_DIR . '/mu-plugins' );
+
+define( 'WP_PLUGIN_DIR', WP_CONTENT_DIR . '/plugins' );
+```
+
+You could require WordPress' default constants functions but that requires a lot more boilerplating to allow those functions to effectively define constants.
 
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