Update to README.md

Just a small edit to clarify the need of psutil.
Tweak for enabling and disabling workers
Completed shots are now marked as such in the database
Merge branch 'master' of https://github.com/fsiddi/brender

# By fsiddi
# Via fsiddi
* 'master' of https://github.com/fsiddi/brender:
  Completed shots are now marked as such in the database
  Tweak for enabling and disabling workers
  Update to README.md

e commit.