Add documentation for tree sheriffs

BUG=skia:
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=1057883002

Review URL: https://codereview.chromium.org/1057883002
diff --git a/site/dev/METADATA b/site/dev/METADATA
new file mode 100644
index 0000000..92294f3
--- /dev/null
+++ b/site/dev/METADATA
@@ -0,0 +1,4 @@
+{
+  "dirOrder": ["chrome", "contrib", "design", "runtime", "testing", "tools", "sheriffing"],
+  "fileOrder": []
+}