Clarify that package manifests must be located at the root of the repository in the Conceptual Overview section of the SwiftPM page (#756)

* Make it explicit that the manifest must be at the root of the package’s repository.

* Revert "Make it explicit that the manifest must be at the root of the package’s repository."

This reverts commit a2a726a.

* Moved the additional information down to a more appropriate place.

* Tweaked copy.

* Moved the paragraph down by one.

Author: daveverwer

documentation/package-manager/_basic-usage.md

@@ -64,6 +64,10 @@ A library is a target that builds a module which can be imported by other packag
 By default, a library module exposes all of the `public` types and methods
 declared in source code located in the `Sources/<target-name>` directory.
 
+When creating a library package intended for use as a dependency in other projects,
+the `Package.swift` manifest must reside at the top level/root of the
+package directory structure.
+
 Run `swift build` to start the Swift build process.
 If everything worked correctly,
 it will compile the Swift module for `PlayingCard`.