warn if no milestone on PR

Summary:
😄
Closes #890

Differential Revision: D5613905

Pulled By: rnystrom

fbshipit-source-id: 1c2b2b46c118741a7b5b84b568a69bf4edb3a199

Dangerfile

@@ -7,12 +7,18 @@ warn("PR is classed as Work in Progress") if github.pr_title.include? "[WIP]"
 # Warn when there is a big PR
 warn("Big PR") if git.lines_of_code > 500
 
-# Changelog entries are required for changes to library files.
+# Changelog entries are required for changes to library files
 no_changelog_entry = !git.modified_files.include?("CHANGELOG.md")
 if has_source_changes && no_changelog_entry && not_declared_trivial && git.lines_of_code > 10
   fail("Any source code changes should have an entry in CHANGELOG.md.")
 end
 
+# Milestones are required to track what's included in each release
+if has_source_changes && not_declared_trivial
+  has_milestone = !github.pr_json['milestone'].nil?
+  warn('All pull requests should have a milestone attached, unless marked *#trivial*.', sticky: false) unless has_milestone
+end
+
 # Docs are regenerated when releasing
 has_doc_changes = !git.modified_files.grep(/docs\//).empty?
 if has_doc_changes