Updates pip install instructions

[eddiebergman]

Previously we had a longer install process but due to the introduction of pre-built wheels, we can install auto-sklearn with a simple pip install auto-sklearn.

[codecov]

Codecov Report

Merging #1237 (b7a4ce4) into development (0f03651) will increase coverage by 0.06%.
The diff coverage is n/a.

❗ Current head b7a4ce4 differs from pull request most recent head 08ec7cd. Consider uploading reports for the commit 08ec7cd to get more accurate results

@@               Coverage Diff               @@
##           development    #1237      +/-   ##
===============================================
+ Coverage        88.22%   88.29%   +0.06%     
===============================================
  Files              139      139              
  Lines            11034    11034              
===============================================
+ Hits              9735     9742       +7     
+ Misses            1299     1292       -7     

Impacted Files	Coverage Δ
autosklearn/pipeline/implementations/util.py	81.25% <0.00%> (-3.13%)	⬇️
autosklearn/evaluation/abstract_evaluator.py	92.12% <0.00%> (-0.79%)	⬇️
autosklearn/ensemble_builder.py	86.43% <0.00%> (+0.60%)	⬆️
autosklearn/util/logging_.py	88.96% <0.00%> (+0.68%)	⬆️
...ine/components/classification/gradient_boosting.py	93.91% <0.00%> (+0.86%)	⬆️
...ipeline/components/regression/gradient_boosting.py	93.26% <0.00%> (+1.92%)	⬆️
...eline/components/feature_preprocessing/fast_ica.py	97.82% <0.00%> (+6.52%)	⬆️

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