You actually bring up a very good point. Almost every other important part of Swift has a manifesto of some sort declaring the goals, commonly rejected items, etc. I wonder if we could get the @core-team to actually work on one or if someone should write a proposal for it. It would be nice to know that there are definite long-term goals for SPM.
This is actually not true (at least partially), there was a "Submit Package" button (or something similar) where you would give it the URL and name of your SPM package and then it would scrape your repo to get the README and other info. I know because once upon a time I used that button to add a few of my own packages.