[Feature Request] Show base branch in Slack open pull request post

[jrodbx]

We're migrating from Bitbucket to Github, and it would be helpful for our release managers + dev team to easily spot the destination/base branch to which a pull request is to be merged, to allow for correction if mistaken initially.

Example of current Bitbucket post:

Example of current Slack post:

[welcome[bot]]

Thanks for opening this issue! If you would like to help implement an improvement, read more about contributing and consider submitting a pull request.

[iamed2]

The contributing link is now dead; is this now closed-source or was development moved elsewhere?