[gnome-online-accounts] facebook: Update README

[Debarshi Ray <debarshir src gnome org>]

commit aab3d1676e5b3ec51f8fb60ef9b4dce838328b73
Author: Debarshi Ray <debarshir gnome org>
Date:   Wed Aug 2 20:34:27 2017 +0200

    facebook: Update README
    
    Mention the Graph API Explorer. It is a handy tool for debugging issues
    with the wire protocol.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=785726

 README |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/README b/README
index a75ddf3..6973165 100644
--- a/README
+++ b/README
@@ -9,6 +9,7 @@ OAuth 2.0:
 https://developers.facebook.com/docs/authentication/
 https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow
 https://developers.facebook.com/docs/reference/dialogs/oauth/
+https://developers.facebook.com/tools/explorer/
 
 Scopes: https://developers.facebook.com/docs/authentication/permissions/