"New package" AutoMerge message should be "friendlier"

[ericphanson]

I think it should start with something like "Welcome to the General registry, and thanks for sharing your work!". That way it is less likely to be perceived as "rude" if it then goes to say a bunch of issues it has with the package.

Ideally the wording should be chosen to still "work" if its their first package or 500th, unless we want to look that up.