Prevent export of .appveyor.yml [minor]

816a220c · Rob Davies · 6b85e52b · 816a220c
Commit 816a220c authored Mar 29, 2018 by Rob Davies
--- a/.gitattributes
+++ b/.gitattributes
@@ -7,6 +7,7 @@
 *.cram    -text diff=cram

 # Omit these files from release tarballs.
+/.appveyor.yml  export-ignore
 .git*           export-ignore
 /.travis.yml    export-ignore
 README.md       export-ignore