Add feature flag

feature-flags.md

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+Why use feature flags?
+======================
+
+* **Velocity**: coupled with a system to rapidly deploy configuration change,
+  it is usually much faster to disable new code by turning off a feature than
+  re-deploying all the code. This extra safety net helps developers be more
+  confident in their release, because they know they can roll back a change
+  very rapidly in case of error.
+* **Testing**: the presence of a feature flag forces the feature owner to test
+  the flow. Without feature flags, the developer might deploy the change and
+  assume the absence of errors means the release was successful. Yet there's
+  numerous failure mode that don't raise explicit errors.
+* **Code iterations**: because code can be kept hidden behind a feature flag
+  until it's ready to go live, developers can push smaller code changes that
+  are not fully integrated yet. Smaller pull requests ease the job of code
+  reviewers, make testing easier, and reduce the probability of a catastrophic
+  failure.
+* **Gradual rollout**: feature flags enable gradual rollout, where a piece of
+  code is gradually activated, for instance on a per city basis, or on a per
+  user group basis. This builds confidence in the feature release process.
+
+References
+----------
+
+* Martin Fowler,
+  [FeatureToggle](http://martinfowler.com/bliki/FeatureToggle.html)
+* Flickr, [Flipping Out](http://code.flickr.net/2009/12/02/flipping-out/): one
+  of the first articles on the topic.
+* [Using Feature Flags to Ship Changes with
+  Confidence](http://blog.travis-ci.com/2014-03-04-use-feature-flags-to-ship-changes-with-confidence/)