added target_project_id to merge_requests.md
added target_project_id to merge_requests.md

Added description about optional field target_project_id for merge requests.
8thcolor/eurucamp2014-htdsadr,8thcolor/eurucamp2014-htdsadr,8thcolor/rubyconfau2015-sadr,8thcolor/rubyconfau2015-sadr,8thcolor/rubyconfau2015-sadr,8thcolor/eurucamp2014-htdsadr