improvements to generated README

* Change "requirements" to "supported platforms" - this is the *only* requirement that a cookbook should bring to the table. All other requirements should be gathered/provided by the cookbook itself. Any other cookbook does not fit the proper pattern.
* Remove narrative around replacing samples with real data. People know this is sample data ;)
* Provide sensible defaults for samples that are more likely to be used
* Use simple markdown format. Not sure who changed this in the first place

This will be PR 1 of a few to help make our cookbook generator make more sense for Berkshelf 3.