[infra] Add documentation to .gitconfig

Change-Id: I3675d36ded67c55bb49e1287f3dca65c6e6b0704
Reviewed-on: https://dart-review.googlesource.com/59089
Reviewed-by: William Hesse <whesse@google.com>
diff --git a/.gitconfig b/.gitconfig
index 7df539b..5a8aa94 100644
--- a/.gitconfig
+++ b/.gitconfig
@@ -1,2 +1,4 @@
+# Add this to your Git config to set the section header as function context for status files.
+# git config --add --local include.path ../.gitconfig
 [diff "status"]
 	xfuncname = "^\\[.*\\]$"