new module docs differ from reality

[garethr]

The README has an example of the modulename of the new module command https://github.com/puppetlabs/pdk#module_name

Required. Specifies the name of the module being created, including the namespace. e.g.: username-my_module

When running with this example the run fails:

pdk new module username-my_module
pdk (FATAL): 'username-my_module' is not a valid module name.
Module names must begin with a lowercase letter and can only include lowercase letters, digits, and underscores.

Relates to #157 in that it doesn't appear possible to specify the namespace.