Add source/docs links to README

Author: ixs

README.md

@@ -11,3 +11,11 @@ Demo Site: https://lto-barcodes.dicp.net
  * FPDF: https://www.fpdf.org
  * OCR Font: https://tsukurimashou.org/ocr.php.en
  * JSBarcode: https://github.com/lindell/JsBarcode
+
+### Documentation
+
+ * https://www.ibm.com/support/pages/ibm-lto-ultrium-cartridge-label-specification
+ * https://www.ibm.com/support/pages/system/files/support/ssg/ssgdocs.nsf/0/c90cc23311a1bfb985256b840063e5cd/$FILE/19P0034,M10321,R00,001,WORDPRO,PDF.pdf
+ * https://am4computers.com/Documents/IBM%20LTO%20Label%20Specification.pdf (Older specs)
+ * https://www.tri-optic.com/media/pdf/RevisedTriOpticBrochure.pdf
+ * https://www.tri-optic.com/media/pdf/ColorTables.pdf