Plugin Directory: Add dates to Changelog section in readme.txt example

[SergeyBiryukov]

Raised by @bethannon1 in ​#forums:

bethannon1: Anyone know why we don't require plugin authors to show the dates of their updates on their changelog? That would be super-handy.... Some put it in, but most don't.
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For instance, because Ninja Forms puts the release date of updates on their changelog, I can tell that the new client site that I'm working in has not been updated since before September 2016. :flushed:

sergey: We could probably pull the dates from SVN, however changelogs are basically freeform text now, like the rest of readme.txt, so automatically displaying the dates there, while theoretically possible, seems like a non-trivial task.
We could educate plugin authors instead and make it a guideline in Plugin Developer Handbook :)
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A good first step would be adding dates to Changelog section in ​readme.txt example.