add hp-75 plotter rom manual

Author: beardicus

README.md

@@ -277,6 +277,7 @@ Sorted alphabetically by company name and product name.
 - [HP 7470A Interconnection Guide](https://archive.org/details/manualzilla-id-7029812)
 - [HP 7470A Operator's Manual (manualslib)](https://www.manualslib.com/manual/1089592/Hp-7470a.html)
 - [HP 7475A Graphics Plotter Operation and Interconnection Manual](https://archive.org/details/HP7475AOperationManual)
+- [HP-75 Plotter ROM External Reference Specification (1982) (PDF)](https://literature.hpcalc.org/community/hp75-plotter-ers.pdf)
 - [HP 7570A DraftPro Plotter Hardware Support Manual](https://archive.org/details/7570adraftproplotterhardwaresupportmanual0757090000201pagesdec86)
 - [HP 7580B Drafting Plotter Service Manual (1986)](https://archive.org/details/hp-7580-b-plotter-service-manual)
 - [HP 7585B Drafting Plotter Service Manual (1983)](https://archive.org/details/bitsavers_hpplotter0_18190273)