Add Gravatar provider lib link (#72)

Author: ottaviano

docs/third-party.md

@@ -8,3 +8,4 @@
 - [`aalaap/faker-youtube`](https://github.com/aalaap/faker-youtube): Faker for YouTube URLs in various formats
 - [`bluemmb/faker-picsum-photos-provider`](https://github.com/bluemmb/Faker-PicsumPhotos): Generate images using [picsum.photos](http://picsum.photos/)
 - [`xvladqt/faker-lorem-flickr`](https://github.com/xvladxtremal/Faker-LoremFlickr): Generate images using [loremflickr.com](http://loremflickr.com/)
+- [`ottaviano/faker-gravatar`](https://github.com/ottaviano/faker-gravatar): Generate avatars using [Gravatar](https://en.gravatar.com/site/implement/images/)