Add a formula for dssp

I had to install the DSSP program. Here is the formula I wrote.

DSSP is a reference tool for assignation of secondary structures
in proteins.

DSSP can be found at http://swift.cmbi.ru.nl/gv/dssp/

If you use DSSP, please cite:

Dictionary of protein secondary structure: pattern recognition of
hydrogen-bonded and geometrical features. Kabsch W, Sander C,
Biopolymers. 1983 22 2577-2637. PMID: 6667333; UI: 84128824.

A series of PDB related databases for everyday needs. Joosten RP, Te
Beek TAH, Krieger E, Hekkelman ML, Hooft RWW, Schneider R, Sander C,
Vriend G, NAR 2010; doi: 10.1093/nar/gkq1105.