clarification to CONTRIBUTING doc

Two things I think need to be clarified:

1. Regarding the symbol that goes into pkgs.janet, should it be the same as the `:name` value in the project's `declare-project` (in the project's project.janet file)?

2. The contributing doc says, "One exception is the janet- prefix in the git repository name to avoid name clashes.", but does not explain the meaning. I *think* what it means to say is, if your git repo name includes the "janet-" prefix, don't include that in the project name. Is that correct?

Start with the CONTRIBUTING document and compare its package naming guidance with the `declare-project` usage in project.janet files and the repository naming convention. Confirm both interpretations raised in the issue, then update the documentation so the symbol, `:name` value, and `janet-` prefix are explained unambiguously.

Written by the indexing model from the issue text.