Add notice regarding the official singing keys

[leonard84]

This change is 

[codecov]

Codecov Report

Merging #984 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #984   +/-   ##
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  Coverage     75.99%   75.99%           
  Complexity     3544     3544           
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  Files           377      377           
  Lines         10788    10788           
  Branches       1374     1374           
=========================================
  Hits           8198     8198           
  Misses         2109     2109           
  Partials        481      481

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[szpak]

It would be good to have those keys added in a commit signed itself with the key which is somehow (https://keybase.io/ ? personal meeting at some conference?) associated with you @leonard84 .

[szpak]

In most of the cases it's better to have it documented in that form than not at all. I created #989 to verify them somehow. Please merge as you wish.