Add instructions to install on apple silicon

[dnerini]

Add instructons to install pysteps on mac computers with apple silicon processors.

@GAaronAlexander, thanks for providing and testing these instructions! Would you mind having a look at this PR and give a feedback?

Closes #334

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (57ece43) 83.34% compared to head (3c04b50) 83.34%.

❗ Current head 3c04b50 differs from pull request most recent head 32d852c. Consider uploading reports for the commit 32d852c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #335   +/-   ##
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  Coverage   83.34%   83.34%           
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  Files         161      161           
  Lines       12355    12355           
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  Hits        10297    10297           
  Misses       2058     2058           

Flag	Coverage Δ
unit_tests	83.34% <ø> (ø)

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[GAaronAlexander]

Hi @dnerini! This looks great and is a clear guide on how to install on apple silicon chips!