Feature request: please consider allowing for custom dictionaries and for the ability to highlight a word >> add to custom dictionary.
I hate seeing all the red squiggly lines underneath the names in my fantasy novel. 
(And yes, I know I can turn off spell checking, but that’s not really the point, is it?)
Although I agree it would be nice if Scrivener directly supported this, there are system methods to take care of the names in your story.
I wrote a blog post specifically about text editing on iOS 9: stavingoffdementia.blogspot.com … ios-9.html
The relevant portion is this:
[i]The Keyboard Dictionary
Auto-Correction maintains a dynamic dictionary of words that are not in the system dictionary. …
You can also add words to the Keyboard Dictionary so that they are not flagged as misspellings, nor Auto-Corrected.
First, you can use Text Replacement. Go to Settings>General>Keyboard>Text Replacement. Add your word or name as a phrase, but leave the shortcut field blank. Now when you type the word it will not be autocorrected.
The second method involves a clever trick using the Contacts app. Names that are present in the Contacts app are not autocorrected. Instead they are treated as if they were in the Keyboard Dictionary. You can add a Contact entry (others have suggested using the last name field of ZZZ for this Contact so that it sorts to the bottom of the Contacts list and is less noticeable. Then, add your new words, using commas to separate them, in the First Name, Last Name (just append to the ZZZ) or Company fields. Using this method has the advantage of allowing you to reset the Keyboard Dictionary without losing all of your special words.
[/i]
I believe that the same techniques work on the Mac. Maybe not ideal, but it does get rid of all the red squiggly highlights.