Add .gitattributes for docs (#3138)

.gitattributes

@@ -0,0 +1,15 @@
+# This file is documented at https://git-scm.com/docs/gitattributes.
+# Linguist-specific attributes are documented at
+# https://github.com/github/linguist.
+
+/vendor/** linguist-vendored
+*.ai binary
+*.pdf binary
+*.png binary
+
+# coverage-excluded is an attribute used to explicitly exclude a path from being
+# included in code coverage. If a path is marked as linguist-generated already,
+# it will be implicitly excluded and there is no need to add the
+# coverage-excluded attribute
+/vendor/** coverage-excluded=true
+/test/** coverage-excluded=true