Add PR checkbox for updating the CHANGELOG
Add PR checkbox for updating the CHANGELOG

Taking this from the Daedalus PR checklist:

https://github.com/input-output-hk/daedalus/blame/79a2044038fef62b020a2587ae2f65738020b9d1/.github/pull_request_template.md#L21
input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype