[data-shapes] Add SHACL-SHACL note content to SHACL Overview doc (#434)

nicholascar has just created a new issue for https://github.com/w3c/data-shapes:

== Add SHACL-SHACL note content to SHACL Overview doc ==
Create content describing SHACL-SHACL - the SHACL validator for SHACL data - and add it to a SHACL Overview document

Depends on https://github.com/w3c/data-shapes/issues/433

Please view or discuss this issue at https://github.com/w3c/data-shapes/issues/434 using your GitHub account


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