add an own chapter regarding signing of packages

[sjuergen]

Signing packages should be an own chapter.
What is a key (private and public key)
How can I create a key (the key should be created once for the complete department/company)
How can I sign the package
How can I as user enable the signature check when I consume packages (Public Key)

also the script in the apax.yml Link should be adapted. The line `apax keygen' has to be removed.

[akzono]

Added this to publishing-lib.md