Adding yahoo finance documentation

[lukashalim]

• closes Simple question about documentation #817
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• passes black --check pandas_datareader
• added entry to docs/source/whatsnew/vLATEST.txt

[lukashalim]

I think this version is better.
I renamed v0.9.1.txt because the file name had an "." in the wrong place, which was causing an error when I tried to build the documentation.

[codecov-commenter]

Codecov Report

Merging #823 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #823      +/-   ##
==========================================
- Coverage   64.67%   64.64%   -0.04%     
==========================================
  Files          62       62              
  Lines        2848     2848              
  Branches      305      305              
==========================================
- Hits         1842     1841       -1     
- Misses        932      933       +1     
  Partials       74       74              

Impacted Files	Coverage Δ
pandas_datareader/tests/test_moex.py	73.91% <0.00%> (-4.35%)	⬇️

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[bashtage]

Can you unstage the sqllite file. This should not be added.

[bashtage]

After that it should be good. The rename is fine with me.

[lukashalim]

I'm unfortunately stuck on unstaging the file.

[lukashalim]

Okay, I think I finally have it - I just went to my fork, deleted cache.sqlite, and committed

[bashtage]

LGTM.