Mark shells crypted from now on

Some info might be leaky, e.g. used versions.
And I guess I'll now start to use them more and more as I finaly start
migration to NixOS.

.gitattributes

@@ -4,3 +4,4 @@ home/files/docker/config.json filter=git-crypt diff=git-crypt
 home/files/rclone.conf filter=git-crypt diff=git-crypt
 home/files/typo3-configuration/client-specific/** filter=git-crypt diff=git-crypt
 home/packages/custom/dmenu-scripts/customer-issue.sh filter=git-crypt diff=git-crypt
+shells/** filter=git-crypt diff=git-crypt