Add documentation for WithCustomStartCell (SpartnerNL#69)

* Add documentation for WithCustomStartCell

* Add code example

Author: GlennM

3.1/exports/collection.md

@@ -181,6 +181,34 @@ class InvoicesExport implements FromCollection, WithStrictNullComparison
 }
 ```
 
+## Custom start cell
+The default start cell is _A1_. Implementing the `WithCustomStartCell` concern in your export class allows you to specify a custom start cell.
+
+```php
+namespace App\Exports;
+
+use App\Invoice;
+use Maatwebsite\Excel\Concerns\FromCollection;
+use Maatwebsite\Excel\Concerns\WithCustomStartCell;
+
+class InvoicesExport implements FromCollection, WithCustomStartCell
+{
+    public function collection()
+    {
+        return Invoice::all();
+    }
+
+    public function startCell()
+    {
+        return 'B2';
+    }
+}
+```
+
+:::warning
+`WithCustomStartCell` is only supported for `FromCollection` exports.
+:::
+
 ## Storing raw contents
 
 If you want to receive the raw contents of the exported file, you can use the `raw()` method:

3.1/exports/concerns.md

@@ -23,7 +23,7 @@ pageClass: no-toc
 |`Maatwebsite\Excel\Concerns\WithCustomCsvSettings`| Allows to run custom Csv settings for this specific exportable. | [Custom CSV Settings](/3.1/exports/custom-csv-settings.html) |
 |`Maatwebsite\Excel\Concerns\WithCharts`| Allows to run one or multiple PhpSpreadsheet Chart instances. | |
 |`Maatwebsite\Excel\Concerns\WithDrawings`| Allows to run one or multiple PhpSpreadsheet (Base)Drawing instances. | [Drawings](/3.1/exports/drawings.html) |
-|`Maatwebsite\Excel\Concerns\WithCustomStartCell`| Allows to specify a custom start cell. Do note that this is only supported for FromCollection exports. | |
+|`Maatwebsite\Excel\Concerns\WithCustomStartCell`| Allows to specify a custom start cell. Do note that this is only supported for FromCollection exports. | [Custom start cell](/3.1/exports/collection.html#custom-start-cell) |
 |`Maatwebsite\Excel\Concerns\WithPreCalculateFormulas`| Forces PhpSpreadsheet to recalculate all formulae in a workbook when saving, so that the pre-calculated values are immediately available to MS Excel or other office spreadsheet viewer when opening the file. | |
 
 ### Traits