[TASK] add XING social icon constants (#2)

Author: vhusaruk92

Configuration/TypoScript/Library/themes.socialmedia.constantsts

@@ -8,6 +8,8 @@
 # themes.configuration.socialmedia.twitterLink = https://twitter.com/typo3
 # themes.configuration.socialmedia.useLinkedIn = 1
 # themes.configuration.socialmedia.linkedInLink = https://www.linkedin.com/company/typo3-association
+# themes.configuration.socialmedia.useXing = 1
+# themes.configuration.socialmedia.xingLink = https://www.xing.com/companies/typo3association
 # themes.configuration.socialmedia.useVimeo = 1
 # themes.configuration.socialmedia.vimeoLink = https://vimeo.com/135225809
 # themes.configuration.socialmedia.useGooglePlus = 1