Remove website2 and CONTRIBUTING.md from release archive files (#3357)

diff --git a/.gitattributes b/.gitattributes
index 65d1ff7..157f783 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,4 +1,6 @@
 # Files and directories with the attribute export-ignore won’t be added to
 # archive files. See http://git-scm.com/docs/gitattributes for details.
 /.gitattributes       export-ignore
+/CONTRIBUTING.md      export-ignore
 /website/             export-ignore
+/website2/            export-ignore