Updates README to not specify g

The current style guide recommends `NSString *text` style syntax except in the case of global string constants. However, this style applies to any constant variable. 

For example, in a non-global constant:
 
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString * const cellIdentifier = @"CellIdentifier";
// Prepare cell
}

Or when using `const` pointers to [prevent re-assignment of a pointer](http://stackoverflow.com/questions/3196491/objective-c-const-nsstring-vs-nsstring-const):

```
NSString * const text = @"New York Times";
text = @"The Economist"; // Illegal; 'Read-only variable is not assignable'
```
```
NSString const *text = @"New York Times";
text = @"The Economist"; // Legal
```