Add gitignore inside example/ (#641)

* Add gitignore inside example/

`"example"` is in the `"files"` key in our package.json, which overrides
the root .gitignore and ends up including the entire examples folder,
including its dist/ and node_modules/. That means a fresh install of
choo comes in at over 100MB of unused code.

Nested .gitignores still apply to folders that are specified in the
`files` key, so this pushes choo back down to a <500KB installed size :)
The example code itself is still included so folks can use it as
offline documentation.

* also ignore package-lock.json

example/.gitignore

@@ -0,0 +1,3 @@
+dist/
+node_modules/
+package-lock.json