added Custom Formatting Values docs

Author: devinfd

3.1/imports/custom-formatting-values.md

@@ -0,0 +1,38 @@
+# Custom Formatting Values
+
+By default Laravel Excel uses PhpSpreadsheet's default value binder to intelligently format a cells value when reading it. You may override this behavior by implementing the `WithCustomValueBinder` concern and the `bindValue` method. Your import class may also extend `PHPExcel_Cell_DefaultValueBinder` to return the default behavior.
+
+```php
+namespace App\Imports;
+
+use PHPExcel_Cell_DataType;
+use PHPExcel_Cell_DefaultValueBinder;
+use PhpOffice\PhpSpreadsheet\Cell\Cell;
+use Maatwebsite\Excel\Concerns\ToModel;
+use Maatwebsite\Excel\Concerns\WithCustomValueBinder;
+
+class UsersImport extends PHPExcel_Cell_DefaultValueBinder implements WithCustomValueBinder, ToModel
+{
+    public function bindValue(Cell $cell, $value)
+    {
+        if (is_numeric($value)) {
+            $cell->setValueExplicit($value, PHPExcel_Cell_DataType::TYPE_NUMERIC);
+
+            return true;
+        }
+
+        // else return default behavior
+        return parent::bindValue($cell, $value);
+    }
+
+    public function model(array $row)
+    {
+        return new User([
+            'name' => $row['0'],
+            'email' => $row['1']
+        ]);
+    }
+}
+```
+
+Available PHPExcel_Cell_DataType's are `TYPE_STRING`, `TYPE_FORMULA`, `TYPE_NUMERIC`, `TYPE_BOOL`, `TYPE_NULL`, `TYPE_INLINE` and `TYPE_ERROR`