Update database_schema.rst

dbsnp updated to version 137
Merge pull request #203 from tanglingfung/patch-3

Update database_schema.rst, thanks to @tanglingfung
Extended PED format added.

The PED file loaded with GEMINI can now have arbitrary columns after the
requisite 6, if a header starting with # is added. The columns in the GEMINI
database will be the names in the header.

For example:
\#family_id sample_id paternal_id maternal_id sex phenotype ethnicity hair_color
1 M10475    None None  1    1    None   brown
1 M10478     M10475  M10500    2    2    None   brown
1 M10500     None    None    2    2    None purple
1 M128215    M10475  M10500    1    1    None blue
Merge branch 'master' of github.com:arq5x/gemini