Update README.md (#628)

Author: siddsarkar

README.md

@@ -47,6 +47,7 @@ These providers implement the OAuth2 spec, but are not OpenID providers, which m
 - [Slack](https://api.slack.com/authentication/oauth-v2) ([Example configuration](./docs/config-examples/slack.md))
 - [Strava](https://developers.strava.com/docs/authentication) ([Example configuration](./docs/config-examples/strava.md))
 - [Spotify](https://developer.spotify.com/documentation/general/guides/authorization-guide/) ([Example configuration](./docs/config-examples/spotify.md))
+- [Unsplash](https://unsplash.com/documentation) ([Example configuration](./docs/config-examples/unsplash.md))
 
 ## Why you may want to use this library