I found the solution. I read a lot of times and always put my `<script src=...>`-code line above the `PlaceHolderAdditionalPageHead` line in my page layout. 

All the answers you provided me actually work. There is just a simple thing what doesn't work (for me). 

I have to put the source-code line **INTO** the `PlaceHolderAdditionalPageHead`-section, not above it. Just tried it out and my test script said: `jQuery library is loaded`