Please add tags corresponding to Yosys releases

[whitequark]

If I remember correctly, @clairexen mentioned that each Yosys release needs to be shipped together with the SymbiYosys master branch commit at the time of release. This however makes it difficult to build historical versions of Yosys, e.g. to bisect changes.

[mmicko]

This is now available https://github.com/YosysHQ/sby/tags

[whitequark]

Thanks! I meant also historically but I guess there is always the YoWASP commit logs if you need that sort of information.