Add Read the Docs badge to README

[jwodder]

I also added a "Documentation" project URL link, which will show up on the left side of the PyPI page.

Closes #826.

[codecov]

Codecov Report

Merging #832 (a898380) into master (1e317cf) will decrease coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #832      +/-   ##
==========================================
- Coverage   85.50%   85.18%   -0.32%     
==========================================
  Files          61       61              
  Lines        6299     6299              
==========================================
- Hits         5386     5366      -20     
- Misses        913      933      +20     

Flag	Coverage Δ
unittests	85.18% <ø> (-0.32%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/support/generatorify.py	0.00% <0.00%> (-16.10%)	⬇️
dandi/dandiarchive.py	86.34% <0.00%> (-1.47%)	⬇️
dandi/download.py	82.21% <0.00%> (-0.54%)	⬇️
dandi/dandiapi.py	89.00% <0.00%> (-0.18%)	⬇️

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[yarikoptic]

thanks!