Updates * naming convention to make exception for any constant

The current README makes an exception to the `NSString *text` convention for global `NSString` constants, but the convention should be generalized to all constants.

For example, in a non-global, static constant:
 ```objc
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString * const cellIdentifier = @"CellIdentifier";
    // Prepare cell
}
```

Or when using `const` to [prevent re-assignment of a pointer](http://stackoverflow.com/questions/3196491/objective-c-const-nsstring-vs-nsstring-const):
```objc
NSString * const authorName = @"Stephen Levy";
text = @"Bret Victor"; // Illegal; 'Read-only variable is not assignable'
```
--
```objc
NSString const *authorName = @"Paul Krugman";
text = @"Max Tagher"; // Legal
```