Add cachetextconv to instructions

Speed up repeated diffs using caching

See https://git-scm.com/docs/gitattributes
[skip ci]

Author: jrafanie

.gitattributes

@@ -3,5 +3,6 @@
 #   2) Add this to ~/.gitconfig:
 #     [diff "gron"]
 #          textconv=gron
+#          cachetextconv=true
 # Now, git log -p, show, and friends will use gron for json differences.
 app/javascript/oldjs/locale/*.json diff=gron