Check in get-pants.sh script, following doc recommendation

[huonw]

This checks in the current get-pants.sh script, following the reasonable recommendation from the docs: https://www.pantsbuild.org/docs/installation

Instead, for regular use, we recommend checking this script into the root of your repo and pointing users and CI machines to that checked-in version

Hopefully having it in this example repo helps encourage users towards this better practice.