Update PDD info in the README.

Author: gkellogg

CONTRIBUTING.md

@@ -28,9 +28,11 @@ This repository uses [Git Flow](https://github.com/nvie/gitflow) to mange develo
   enough, be assured we will eventually add you in there.
 * Do note that in order for us to merge any non-trivial changes (as a rule
   of thumb, additions larger than about 15 lines of code), we need an
-  explicit [public domain dedication][PDD] on record from you.
+  explicit [public domain dedication][PDD] on record from you,
+  which you will be asked to agree to on the first commit to a repo within the organization.
+  Note that the agreement applies to all repos in the [Ruby RDF](https://github.com/ruby-rdf/) organization.
 
 [YARD]:    https://yardoc.org/
 [YARD-GS]: https://rubydoc.info/docs/yard/file/docs/GettingStarted.md
-[PDD]:     https://lists.w3.org/Archives/Public/public-rdf-ruby/2010May/0013.html
-[pr]:      https://github.com/ruby-rdf/json-ld/compare/
+[PDD]:              https://unlicense.org/#unlicensing-contributions
+[pr]:      https://github.com/ruby-rdf/rdf/compare/

README.md

@@ -552,7 +552,9 @@ To get a local working copy of the development repository, do:
   list in the the `README`. Alphabetical order applies.
 * Do note that in order for us to merge any non-trivial changes (as a rule
   of thumb, additions larger than about 15 lines of code), we need an
-  explicit [public domain dedication][PDD] on record from you.
+  explicit [public domain dedication][PDD] on record from you,
+  which you will be asked to agree to on the first commit to a repo within the organization.
+  Note that the agreement applies to all repos in the [Ruby RDF](https://github.com/ruby-rdf/) organization.
 
 License
 -------
@@ -564,7 +566,7 @@ see <https://unlicense.org/> or the accompanying {file:UNLICENSE} file.
 [RDF]:              https://www.w3.org/RDF/
 [YARD]:             https://yardoc.org/
 [YARD-GS]:          https://rubydoc.info/docs/yard/file/docs/GettingStarted.md
-[PDD]:              https://lists.w3.org/Archives/Public/public-rdf-ruby/2010May/0013.html
+[PDD]:              https://unlicense.org/#unlicensing-contributions
 [RDF.rb]:           https://rubygems.org/gems/rdf
 [Rack::LinkedData]: https://rubygems.org/gems/rack-linkeddata
 [Backports]:        https://rubygems.org/gems/backports