Update README.md file for new config file attributes

Since the code has changed to add the mongodb configuration options
into the mongodb[:config] attribute, the documentation had to be
updated. This relates to issues #229 and #182
Replace old configuration files attributes to new mongodb[:config] notation

Since the code was refactored to use the mongodb[:config] attribute to
store configuration settings, setting the attributes in a wrapper
cookbook was not working. This commit replaces all references to the
new notation. See #182