Add zh_HANS language mapping to pdfjs and EDX_README

common/static/js/vendor/pdfjs/EDX_README

+Please keep this document up to date with any changes made to pdfjs in our repo so we can carry them forward if/when we update the package.
+
+2017-09-01 - Made a reference for zh-hans in locale.properties to the zh-cn translations in preparation for a Django upgrade from 1.8 - 1.11 that removes their use of zh-cn.

common/static/js/vendor/pdfjs/locale/locale.properties

@@ -304,6 +304,9 @@
 [zh-CN]
 @import url(zh-CN/viewer.properties)
 
+[zh-HANS]
+@import url(zh-CN/viewer.properties)
+
 [zh-TW]
 @import url(zh-TW/viewer.properties)