Add docs and an example for add_vba_project.

Issue #29

examples/macro.c

@@ -0,0 +1,25 @@
+/*****************************************************************************
+ *
+ * An example of adding macros to a libxlsxwriter file using a VBA project
+ * file extracted from an existing Excel .xlsm file.
+ *
+ * The vba_extract.py utility from the libxlsxwriter examples directory can be
+ * used to extract the vbaProject.bin file.
+ *
+ * Copyright 2014-2018, John McNamara, jmcnamara@cpan.org
+ */
+
+#include "xlsxwriter.h"
+
+int main() {
+
+    lxw_workbook  *workbook  = new_workbook("macro.xlsm");
+    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
+
+    /* Add a macro that will execute when the file is opened. */
+    workbook_add_vba_project(workbook, "vbaProject.bin");
+
+    worksheet_write_string(worksheet, 0, 0, "Overwrite this", NULL);
+
+    return workbook_close(workbook);
+}