minor changes in ABNF
Merge pull request #3 from nichtich/patch-1

minor changes in ABNF
minor: typos and changed MARC spec --> MARCspec
minor: another example for X in control fields
edited introduction and order of sections
clearer defined terminology
interpretation of omitted indicators
minor: link
redefined data content
data content see data element value
'~' is now '/' and 'X' is now '.'
new indizes, subfieldtags etc.
minor changes in hierarchy and wording
Merge pull request #11 from nichtich/patch-2

minor changes in hierarchy and wording
Update README.md
open end character postion or range
Create marc-spec-parser-rules.md
Update marc-spec-parser-rules.md
allow charpos on subfields
modification of subfieldSpec