[MEDIUM] errorfile: use a local file to feed error messages

It is now possible to read error messages from local files,
using the 'errorfile' keyword. Those files are read during
parsing, so there's no I/O involved. They make it possible
to return custom error messages with custom status and headers.
diff --git a/examples/errorfiles/README b/examples/errorfiles/README
new file mode 100644
index 0000000..a882632
--- /dev/null
+++ b/examples/errorfiles/README
@@ -0,0 +1,9 @@
+These files are default error files that can be customized
+if necessary. They are complete HTTP responses, so that
+everything is possible, including using redirects or setting
+special headers.
+
+They can be used with the 'errorfile' keyword like this :
+
+    errorfile 503 /etc/haproxy/errors/503.http
+