Change Usage section of the README.md

[vitonzhangtt]

My project is in Objective-C.

I got an error "'UITextView+Placeholder/UITextView+Placeholder.h' file not found" after importing this library by 'pod install'.

Then I found that the right way to import "<UITextView_Placeholder/UITextView+Placeholder.h>".
Expect to add another item in the Usage section; like the following:

Import Dynamic Framework:

e.g. If you're using CocoaPods with use_frameworks! flag.

@import UITextView_Placeholder;
Import Static Library:

#import <UITextView+Placeholder/UITextView+Placeholder.h>

Import by pod:
#import <UITextView_Placeholder/UITextView+Placeholder.h>

[devxoul]

@vitonzhangtt you can use @import UITextView_Placeholder; when using use_framework!. It is documented on README.md