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 manage devel
   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/rdf-spec/compare/
+[PDD]:              https://unlicense.org/#unlicensing-contributions
+[pr]:      https://github.com/ruby-rdf/rdf/compare/

README.md

@@ -76,7 +76,9 @@ follows:
   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.
 
 License
 -------
@@ -87,4 +89,4 @@ see <https://unlicense.org/> or the accompanying {file:UNLICENSE} file.
 [RDF.rb]:    https://rubygems.org/gems/rdf
 [RSpec]:     https://rspec.info/
 [RubySpec]:  https://rubyspec.org/wiki/rubyspec/Style_Guide
-[PDD]:       https://lists.w3.org/Archives/Public/public-rdf-ruby/2010May/0013.html
+[PDD]:              https://unlicense.org/#unlicensing-contributions