Slack: Update documentation to indicate that the project no longer accepts PRs

[smbkr]

As per @ashokirla's comment in a recent PR, this project no longer accepts PRs or contributions from the open source community. The documentation (README.md, contributing.md) should be updated to reflect this, since there are a lot of open PRs & it would be best for people to be aware of this before putting their time into working on contributions.

The welcome bot message (see below) should be updated also.

[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.