Add note to language/informal deprecating the directory

Change-Id: I2b2d564f994f5680608607b369a7b2b45d8a2154
Reviewed-on: https://dart-review.googlesource.com/69640
Reviewed-by: Leaf Petersen <leafp@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
diff --git a/docs/language/informal/README.md b/docs/language/informal/README.md
index 5774998..ed534fb 100644
--- a/docs/language/informal/README.md
+++ b/docs/language/informal/README.md
@@ -1,5 +1,9 @@
 This directory contains "informal specifications".
 
+**Note**: This directory is no longer where new language proposals are
+discussed. Instead, new language proposals should be submitted to
+[dart-lang/language](https://github.com/dart-lang/language).
+
 In order to move faster and get better feedback, we implement and iterate on
 language changes before the full official specification has been written. Still,
 the implementers need *something* to go on.