.hgignore: add comment setting policy

R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/44810044
diff --git a/.hgignore b/.hgignore
index 571db5f..c27ee1e 100644
--- a/.hgignore
+++ b/.hgignore
@@ -1,2 +1,3 @@
+# Add no patterns to .hgignore except for files generated by the build.
 syntax:glob
 last-change