Add release v0.12.0 to the changelog

Author: LefterisJP

docs/changelog.rst

@@ -2,7 +2,8 @@
 Changelog
 =========
 
-* :feature:`2699` Add ``/channels/<token_address>`` REST-API endpoint to query all node's channels for a specific token
+* :release:`0.12.0 <2018-10-05>`
+* :feature:`2699` Add ``/channels/<token_address>`` REST-API endpoint to query all node's channels for a specific token.
 * :feature:`2568` Validate the state changes for the Delivered and Processed sender.
 * :bug:`2567` Increase default channel reveal timeout to 50 blocks.
 * :bug:`2676` Return an error if an invalid ``joinable_funds_target`` value is provided to the connect endpoint.