Add a second link to the docs from the README

[timholy]

The badges seem less visible to some people than text links (I may have been in that camp once myself?), so let's have both.

Closes #587

[codecov[bot]]

Codecov Report

Merging #589 (5737024) into master (6da0b26) will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
+ Coverage   86.63%   86.65%   +0.01%     
==========================================
  Files          12       12              
  Lines        2537     2533       -4     
==========================================
- Hits         2198     2195       -3     
+ Misses        339      338       -1     

see 3 files with indirect coverage changes