Fixes to the bsdl-gpl article

[salvadore]

Summary

Fixes a few things in the bsdl-gpl article:

• inconsistency regarding AT&T at the beginning of the text, which later becomes ATT: I choosed to always use AT&T;
• style improvement: change "adds additional" into "adds more";
• change "Addenda" into "Bibliographical References", since that is what is in the section;
• fix typos.

[carlavilla]

@bcr please can you take a look at this changes? (For me everything it's correct)

If everything it's correct I can make the commit myself

[bcr]

Good to go!