Include information about the bug database in gn's README.md.

Change-Id: Iacbbaa8dea7b24bdf0635dcad80797acecf93922
Reviewed-on: https://gn-review.googlesource.com/c/3020
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>

README.md

diff --git a/README.md b/README.md
index cc981fa..7e09117 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,11 @@
 expected to be found in `PATH`. This can be overridden by setting `CC`, `CXX`,
 and `AR`.
 
+## Reporting bugs
+
+If you find a bug, you can see if it is known or report it in the [bug
+database](https://bugs.chromium.org/p/gn/issues/list).
+
 ## Sending patches
 
 GN uses [Gerrit](https://www.gerritcodereview.com/) for code review. The short