document build trigger env var (#611)

Co-authored-by: dependabot-ci <[email protected]>

Author: TP Honey

content/pipeline/environment/reference/drone-build-trigger.md

@@ -0,0 +1,14 @@
+---
+date: 2000-01-01T00:00:00+00:00
+title: DRONE_BUILD_TRIGGER
+author: tphoney
+---
+
+The source of the trigger for the build. eg a user, or webhook (@hook).
+
+```
+DRONE_BUILD_TRIGGER=root
+DRONE_BUILD_TRIGGER=f.bar
+DRONE_BUILD_TRIGGER=@hook
+DRONE_BUILD_TRIGGER=tphoney
+```