Add JuliaHub version badge to README.md

[waldyrious]

Since TagBot doesn't seem to be set up correctly and is not generating tags based on the released versions (@giordano do you know if this is still a TagBot issue rather than merely the repo being misconfigured?), I thought it would be useful to add a version badge to the README to at least provide the version information more prominently.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.23%. Comparing base (a74455e) to head (25f1cea).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #59   +/-   ##
=======================================
  Coverage   95.23%   95.23%           
=======================================
  Files           1        1           
  Lines          21       21           
=======================================
  Hits           20       20           
  Misses          1        1           

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[giordano]

do you know if this is #41 (comment) rather than merely the repo being misconfigured?

GitHub wiped out the Documenter key, which was used by TagBot to publish the release. I recreated it and now we have
https://github.com/JuliaMath/Tau.jl/releases/tag/v2.0.1

[waldyrious]

Nice! Thanks :)

I'll go ahead and merge this, which offers a nice visual indicator in addition to GitHub's releases.