Add knockknock

[setu4993]

Checklist

• License file is packaged (see here for an example)
• Source is from official source
• Package does not vendor other packages. (If a package uses the source of another package, they should be separate packages or the licenses of all packages need to be packaged)
• If static libraries are linked in, the license of the static library is packaged.
• Build number is 0
• A tarball (url) rather than a repo (e.g. git_url) is used in your recipe (see here for more details)
• GitHub users listed in the maintainer section have posted a comment confirming they are willing to be listed there

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/knockknock) and found it was in an excellent condition.

[setu4993]

@conda-forge/staged-recipes, @conda-forge/help-python : This PR is ready for review.