Use LN abbreviation for Lightning Network (glossary file)

[yahiheb]

As suggested in #36

[renepickhardt]

I am not sure about the use of that abbreviation in general. But I think in the glossary we should not do it as the O'Reilly style is that the first time a word appears it is written out and then abbreviated for the remainder of that section However in a glossary people usually don't reed from alpha through omega but rather look up single terms. Thus I would rather reject this PR. any thoughts @aantonop

[aantonop]

Agreed. Let's not do it in the glossary or appendixes