My go on the ERD

I've showed it to Joana and she said is good. It shouldn't be bigger
than this or it will get too complex for the 2 start schemas we have to
complete.
Merge branch 'master' of https://github.com/criscmaia/BIS3214

# By Cristiano Maia
# Via Cristiano Maia
* 'master' of https://github.com/criscmaia/BIS3214:
  My go on the ERD