Documentation fixes

[deepak]

• explain why do we need scale 4 in total_amount @gnufied says to store fractional currency ie. pennies and to store more precision for calculations with tax rate
• generator command no longer works. in http://ept.github.io/invoicing/getting_started.html
• in http://ept.github.io/invoicing/getting_started.html can use acts_as_invoice acts_as_credit_note etc rather than acts_as_ledger_item :subtype => :invoice

[pmaojo]

That seems to be another project. http://ept.github.io/invoicing vs https://github.com/code-mancers/invoicing/issues/26