Add information about having equal signs align

There was not source of truth whether to align equal signs or not
in our code. This should help people not align equal signs as well
as help them rethink their code if they have many assignments.
I also agree... if anything I would say we should mention that it's probably a bad idea to have too many assignments as a foot note:

```
# bad
def something
  foo     = bar
  humdrum = an_operation
  pi      = 3.14159265359
  vi      = "smells"
end

# good
def something
  foo = bar
  humdrum = an_operation
  pi = 3.14159265359
  vi = "smells"
end
```

NOTE: There's a good chance that too many assignments is a smell of another problem - see here (link to other rule) for a more detailed example.