Update PDD info in the README.

Author: gkellogg

README.md

@@ -150,8 +150,31 @@ RDF.rb Compatibility
 
 As of version 2.0, this software depends on RDF.rb 3.1 or greater.
 
-
-License
-========
+## Contributing
+
+This repository uses [Git Flow](https://github.com/nvie/gitflow) to mange development and release activity. All submissions _must_ be on a feature branch based on the _develop_ branch to ease staging and integration.
+
+* Do your best to adhere to the existing coding conventions and idioms.
+* Don't use hard tabs, and don't leave trailing whitespace on any line.
+  Before committing, run `git diff --check` to make sure of this.
+* 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` or `VERSION` 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.
+* Don't touch the `AUTHORS` file. If your contributions are significant
+  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,
+  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 software is released under a public domain waiver (Unlicense).
+
+[YARD]:             https://yardoc.org/
+[YARD-GS]:          https://rubydoc.info/docs/yard/file/docs/GettingStarted.md
+[PDD]:              https://unlicense.org/#unlicensing-contributions