[osis-core] minor manual addition

[Chris Little]

Another minor thing to add to the manual--

In Appendix B.1, in the script attribute description, some mention 
should be made that codes should conform to ISO 15924 
(http://www.unicode.org/iso15924/iso15924-codes.html). This is the 
standard we based our regex on and, since script was initially added to 
OSIS, the standard has actually been completed.

--Chris