CI badge, correct Julia requirement to 1.6

[stevengj]

Add CI badge to README

And test Julia 1.3 since that is the earliest supported version according to Project.toml … Hmm, it seems that Julia 1.3 is not actually supported any more, since xsum_jll now requires Julia 1.6. (Note that changing the Project.toml in this way does not actually affect downstream packages, since earlier versions of Julia could not install this version of Xsum anyway.)

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.14 🎉

Comparison is base (4ce13a0) 87.80% compared to head (fcd3b53) 87.95%.

❗ Current head fcd3b53 differs from pull request most recent head 3c08a79. Consider uploading reports for the commit 3c08a79 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
+ Coverage   87.80%   87.95%   +0.14%     
==========================================
  Files           2        2              
  Lines          82       83       +1     
==========================================
+ Hits           72       73       +1     
  Misses         10       10              

see 1 file with indirect coverage changes

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