Added context manager example.
Added context manager example.

jvrsantacruz/XlsxWriter,jvrsantacruz/XlsxWriter,jmcnamara/XlsxWriter,Thhhza/XlsxWriter,gechong/XlsxWriter,mdeemer/XlsxWriter,seize-the-dave/XlsxWriter,liukaijv/XlsxWriter,mdeemer/XlsxWriter,Thhhza/XlsxWriter,applicationdevm/XlsxWriter,seize-the-dave/XlsxWriter,seize-the-dave/XlsxWriter,Thhhza/XlsxWriter,jkyeung/XlsxWriter,liukaijv/XlsxWriter,Thhhza/XlsxWriter,jvrsantacruz/XlsxWriter,yaojingwu1992/XlsxWriter,jkyeung/XlsxWriter,jmcnamara/XlsxWriter,applicationdevm/XlsxWriter,gechong/XlsxWriter,jkyeung/XlsxWriter,gechong/XlsxWriter,liukaijv/XlsxWriter,yaojingwu1992/XlsxWriter,yaojingwu1992/XlsxWriter,mdeemer/XlsxWriter,applicationdevm/XlsxWriter,jmcnamara/XlsxWriter,liukaijv/XlsxWriter,mdeemer/XlsxWriter,jkyeung/XlsxWriter,jvrsantacruz/XlsxWriter,applicationdevm/XlsxWriter,seize-the-dave/XlsxWriter,gechong/XlsxWriter,yaojingwu1992/XlsxWriter,jmcnamara/XlsxWriter