Update layer docs for packages.el to note where should local packages reside
Update layer docs for packages.el to note where should local packages reside

The information about location of local packages is noted in the parent section. While this can be considered enough I kept re-reading `packages.el` and couldn't find the information. Adding the information here as well makes the section more self-contained.

tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs