Add link to PackageDescription API to README.md (#2685)

* Add link to PackageDescription API to README.md

I don't think this link is present anywhere else in the repository, but it can be tremendously useful to users when working with Package.swift files.

* Add period to the new README.md link

Co-Authored-By: Kyle Murray <krilnon@users.noreply.github.com>

Co-authored-by: Kyle Murray <krilnon@users.noreply.github.com>

README.md

@@ -37,6 +37,8 @@ For extensive documentation on using Swift Package Manager, creating packages, a
 
 For additional documentation on developing the Swift Package Manager itself, see [Documentation/Development](Documentation/Development.md).
 
+For detailed documentation on the package manifest API, see [PackageDescription API](https://docs.swift.org/package-manager/PackageDescription/index.html).
+
 ---
 
 ## System Requirements