Twenty Fifteen: Add Foursquare icon support to the social link menu.

props iamtakashi.
fixes #30100.

git-svn-id: https://develop.svn.wordpress.org/trunk@30023 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-content/themes/twentyfifteen/readme.txt

@@ -53,6 +53,7 @@ Available icons: (Linking to any of the following sites will automatically displ
 * Dropbox
 * Facebook
 * Flickr
+* Foursquare
 * GitHub
 * Google+
 * Instagram

src/wp-content/themes/twentyfifteen/style.css

@@ -781,6 +781,10 @@ a:focus {
 	content: "\f211";
 }
 
+.social-navigation a[href*="foursquare.com"]:before {
+	content: "\f226";
+}
+
 .social-navigation a[href*="plus.google.com"]:before {
 	content: "\f206";
 }