Adding official reference for BBS signature scheme variant

[feidens]

Add official reference for BBS signature scheme variant to libraries.md. We can additionally cite the thesis.

@InProceedings{SCN:AuSusMu06,
  author =       "Man Ho Au and
                  Willy Susilo and
                  Yi Mu",
  title =        "Constant-Size Dynamic k-{TAA}",
  pages =        "111--125",
  editor =       scn06ed,
  booktitle =    scn06name,
  volume =       scn06vol,
  address =      scn06addr,
  month =        scn06month,
  publisher =    scnpub,
  series =       mylncs,
  year =         2006,
  doi =          "10.1007/11832072_8",
}