Add CONTRIBUTING.md.

Author: gkellogg

CONTRIBUTING.md

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+# How to contribute
+
+Community contributions are essential for keeping Ruby RDF great. We want to keep it as easy as possible to contribute changes that get things working in your environment. There are a few guidelines that we need contributors to follow so that we can have a chance of keeping on top of things.
+
+## Development
+
+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.
+
+* create or respond to an issue for on the [Github Repository](http://github.com/ruby-rdf/json-ld/issues)
+* Fork and clone the repo:
+  `git clone git@github.com:your-username/json-ld.git`
+* Install bundle:
+  `bundle install`
+* Create tests in RSpec and make sure you achieve at least 90% code coverage for the feature your adding or behavior being modified.
+* Push to your fork and [submit a pull request][pr].
+
+## Do's and Dont's
+* 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.
+
+[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/json-ld/compare/