Updated README with a migration example for allowing nills

If you use the migration helper for an allow_nil field, it takes a bit of digging to work out how to do it. The normal approach of `null: false` doesn't work.

I've added a working example to the section on allowing nils to make it a bit clearer:

```ruby
add_money :products, :optional_price, amount: { null: true, default: nil }
```
bump money.gem requirement to 6.0
update contributors
version bump to 0.9.0
Merge pull request #122 from mcoms/patch-1

Updated README with a migration example for allowing nills
Merge pull request #144 from RubyMoney/dep-updates

update deps
Allow per-attribute disabling of validations