Add changelog instructions to CHANGELOG.md

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CHANGELOG.md

-Please view this file on the master branch, on stable branches it's out of date.
+**Note:** This file is automatically generated. Please see the [developer
+documentation](doc/development/changelog.md) for instructions on adding your own
+entry.
 ## 8.14.0 (2016-11-22)
+
 - Fix Milestone dropdown not stay selected for `Upcoming` and `No Milestone` option !7117
 - Backups do not fail anymore when using tar on annex and custom_hooks only. !5814
 - Adds user project membership expired event to clarify why user was removed (Callum Dryden)