Money#dollars & Money#amount return BigDecimal

Money#dollars and Money#amount methods return numbers of type BigDecimal
rather than Float. Floats have esoteric precision qualities that make
them a bad match for representing values of currency. The Money gem
should set an example and promote the use of BigDecimal.
Comments describe Money#amount returning BigDecimal
Post install message for #amount and #dollars

A warning for people upgrading to version 6 that the Money#amount and
Money#dollars methods now return BigDecimal values.
Migration notes listed in the README

We document the API changes here. First up is the change of
Money#dollars and Money#amount methods returning BigDecimal values.
Reallign documentation
Update README.md

Add warning to read the migration notes when upgrading to a major version
Update README.md

Fix link to the migration notes
Update README.md
Merge pull request #311 from orien/master

Money#dollars and Money#amount methods return BigDecimal values