checksumuri — Give the uri to a file that contains the checksum for this package.
When a package is created with the ReST packager a checksum is printed,
which can be placed in a file on a webspace. This element points to such a file
to give the ReST application the ability to verify the package before using it.
This behavior is optional. ReST packages already contain checksums of the
files contained, which are verified when a package is used; package checksums
are in addition to this behavior. At the current time
checksumuri is not supported, but will be added to future
versions of ReST.