Add Readeck Button to README

[Joedmin]

Hello, I've created a new extention called Readeck Button and would love to show it to the world

[Alkarex]

Hello,
You could consider adding a line for your extension to https://github.com/FreshRSS/Extensions/blob/master/repositories.json
so that it gets shown inside FreshRSS along version updates.

[Alkarex]

@Joedmin You could consider updating the automated tests for your extension, on the model of the tests in https://github.com/FreshRSS/Extensions/tree/master to catch warnings, deprecations, etc. In particular PHPStan.

[Joedmin]

Thank you for your tip!