Added changelog.md

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

CHANGELOG.md

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+ChangeLog
+=========
+
+NC 11 (????-??-??)
+------------------
+* PHP 5.4 and 5.5 no longer supported
+* PHP 7.1 support
+* OC_L10N removed use \OCP\IL10N (#1948)