README command-line example

I think the examples should be copy-pastable and use the same values as other suggestions in the README. For instance, the README states to ‘make a directory called "schemas"’ and the default place for assets is the `resources` dir. Therefore I would suggest a version like so:

```
$ xcdm MyApplicationName ./schemas ./resources
```
#

A second issue is that the command seems to require 4 arguments, of which I believe the last one is the ‘Xcode application version’.

```
/Library/Ruby/Gems/2.0.0/gems/ruby-xcdm-0.0.7/lib/xcdm/schema.rb:87:in `initialize': wrong number of arguments (3 for 4) (ArgumentError)
from /Library/Ruby/Gems/2.0.0/gems/ruby-xcdm-0.0.7/bin/xcdm:6:in `new'
from /Library/Ruby/Gems/2.0.0/gems/ruby-xcdm-0.0.7/bin/xcdm:6:in `'
from /usr/bin/xcdm:23:in `load'
from /usr/bin/xcdm:23:in `'
```

This is what I ended up using:

```
$ bundle exec xcdm MyApplicationName ./schemas ./resources 5.1
```

Start with the README command example, then inspect bin/xcdm and lib/xcdm/schema.rb around the reported argument error. Run the shown commands to confirm the required arguments and compare them with the README's directory names. Done means the documented command is copy-pastable and accurately explains the Xcode version argument.

Written by the indexing model from the issue text.