Add note about documentation

Copied from [Poltergeist](https://github.com/teampoltergeist/poltergeist/blob/master/README.md) because it is a fantasic idea 👏
collectiveidea · Aug 14, 2015 · 7e6e5f7 · 7e6e5f7
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+**If you're viewing this at https://github.com/collectiveidea/delayed_job,
+you're reading the documentation for the master branch.
+[View documentation for the latest release
+(4.0.6).](https://github.com/collectiveidea/delayed_job/tree/v4.0.6)**
+
 Delayed::Job
 ============
 [![Gem Version](https://badge.fury.io/rb/delayed_job.png)][gem]