Added doc for hook_civicrm_entityRefFilters

For https://github.com/civicrm/civicrm-core/pull/15231

docs/hooks/hook_civicrm_entityRefFilters.md

+# hook_civicrm_entityRefFilters
+
+## Summary
+
+This hook is called when filters and create links for entityRef field is build.
+
+## Definition
+
+    hook_civicrm_entityRefFilters(&$filters, &$links)
+
+## Parameters
+
+-   array $filters - reference to list of filters
+-   array $links - reference to list of create links
+
+## Returns
+
+## Example
+
+     /**
+      * Implements hook_civicrm_entityRefFilters().
+      *
+      * @link http://wiki.civicrm.org/confluence/display/CRMDOC/hook_civicrm_entityRefFilters
+      */
+     function modulename_civicrm_entityRefFilters(&$filters, &$links) {
+       // Add New Staff link on entityRef field of contact
+       $links['Contact'][] = [
+         'label' => ts('New Staff'),
+         'url' => CRM_Utils_System::url('/civicrm/profile/create', 'reset=1&context=dialog&gid=5'),
+         'type' => 'Individual',
+         'icon' => 'fa-user',
+       ];
+
+       // Add Do not email filter on contact entity ref field.
+       $filters['Contact'][] = [
+         'key' => 'do_not_email',
+         'value' => ts('Do Not Email'),
+       ];
+       // Add Marital status filter on contact entity ref field.
+       $filters['Contact'][] = [
+         'key' => 'custom_2',
+         'value' => ts('Marital status'),
+       ];
+
+       // Add custom field of address as filter on contact entity ref field.
+       $filters['Contact'][] = [
+         'key' => 'custom_34',
+         'value' => ts('Belongs to'),
+         'entity' => 'Address',
+       ];
+     }