Updating README

Added a bit more information to the install procedure, since it was using an uppercase `Carbon` which breaks some of composer's new tooling and added a note for users to avoid using `*`

I did not run `readme.php` as it broke many things like the "maxValue" examples.
@GrahamCampbell, yes it is.

```
$ composer require nesbot/carbon
Using version ~1.13 for nesbot/carbon
```

I was as surprised as you were when i found out.