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]:    http://yardoc.org/
-[YARD-GS]: http://rubydoc.info/docs/yard/file/docs/GettingStarted.md
-[PDD]:     http://lists.w3.org/Archives/Public/public-rdf-ruby/2010May/0013.html
-[pr]:      https://github.com/ruby-rdf/ld-patch/compare/
+[YARD]:    https://yardoc.org/
+[YARD-GS]: https://rubydoc.info/docs/yard/file/docs/GettingStarted.md
+[PDD]:              https://unlicense.org/#unlicensing-contributions
+[pr]:      https://github.com/ruby-rdf/rdf/compare/

README.md

@@ -94,7 +94,11 @@ This repository uses [Git Flow](https://github.com/nvie/gitflow) to mange develo
 * Do document every method you add using [YARD][] annotations. Read the [tutorial][YARD-GS] or just look at the existing code for examples.
 * Don't touch the `.gemspec`, `VERSION` or `AUTHORS` files. If you need to change them, do so on your private branch only.
 * Do feel free to add yourself to the `CREDITS` file and the corresponding 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.
+* 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,
+  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
 This is free and unencumbered public domain software. For more information,
@@ -106,7 +110,7 @@ A copy of the [LD Patch EBNF](file:etc/ld-patch.ebnf) and derived parser files a
 [RDF]:            http://www.w3.org/RDF/
 [YARD]:           http://yardoc.org/
 [YARD-GS]:        http://rubydoc.info/docs/yard/file/docs/GettingStarted.md
-[PDD]:             http://lists.w3.org/Archives/Public/public-rdf-ruby/2010May/0013.html
+[PDD]:              https://unlicense.org/#unlicensing-contributions
 [RDF.rb]:         https://rubygems.org/gems/rdf
 [RDF::XSD]:       https://rubygems.org/gems/rdf-xsd
 [EBNF]:           https://rubygems.org/gems/ebnf