DOC: Update index

[bashtage]

Stop using README and prefer a custom index

• closes #xxxx
• 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

[codecov-commenter]

Codecov Report

Merging #804 into master will decrease coverage by 19.82%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #804       +/-   ##
===========================================
- Coverage   84.59%   64.76%   -19.83%     
===========================================
  Files          62       62               
  Lines        2810     2810               
  Branches      301      301               
===========================================
- Hits         2377     1820      -557     
- Misses        360      918      +558     
+ Partials       73       72        -1     

Impacted Files	Coverage Δ
pandas_datareader/iex/ref.py	0.00% <0.00%> (-88.89%)	⬇️
pandas_datareader/iex/stats.py	0.00% <0.00%> (-81.70%)	⬇️
pandas_datareader/tests/av/test_av_time_series.py	29.89% <0.00%> (-70.11%)	⬇️
pandas_datareader/av/forex.py	27.65% <0.00%> (-68.09%)	⬇️
pandas_datareader/tests/test_utils.py	35.71% <0.00%> (-64.29%)	⬇️
pandas_datareader/tiingo.py	33.66% <0.00%> (-62.38%)	⬇️
pandas_datareader/quandl.py	30.95% <0.00%> (-61.91%)	⬇️
pandas_datareader/tests/test_oecd.py	37.50% <0.00%> (-59.38%)	⬇️
pandas_datareader/oecd.py	37.50% <0.00%> (-58.34%)	⬇️
pandas_datareader/tests/test_tiingo.py	42.30% <0.00%> (-57.70%)	⬇️
... and 17 more

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