Add Watch The Difference as a zsh til.

zsh/watch-the-difference.md

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+# Watch The Difference
+
+The `watch` command is a simple way to repeatedly run a particular command.
+I'll sometimes use it to monitor the response from some endpoint. `watch`
+can make monitoring responses even easier when the `-d` flag is employed.
+This flag instructs `watch` to highlight the parts of the output that are
+*different* from the previous run of the command.
+
+So if I run
+
+```
+$ watch -d curl -LIs localhost:3000
+```
+
+I can easily see if the http status of the request changes.