Update json differ to use gron

Provide instructions to install and enable gron for human friendly
json differences.

I used gron to review #9366 and found it easy to setup and use.

[skip ci]

Author: jrafanie

.gitattributes

@@ -1 +1,7 @@
-app/javascript/oldjs/locale/*.json -text -diff
+# To use gron for human readable json differences:
+#   1) brew/apt install gron
+#   2) Add this to ~/.gitconfig:
+#     [diff "gron"]
+#          textconv=gron
+# Now, git log -p, show, and friends will use gron for json differences.
+app/javascript/oldjs/locale/*.json diff=gron