Update README for mpremote and package usage

[brainelectronics]

This change updates the manual package installation instructions to use mpremote over rshell.

In the PyPI section the usage of twine check before twine upload is recommended.

An example instruction on how to install a specific version with upip was added.

The package usage example and the corresponding main.py example files are extended to set the LED pin based on the board extracted from os.uname().

[codecov-commenter]

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@@            Coverage Diff            @@
##              main       #35   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines           11        11           
  Branches         1         1           
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  Hits            11        11           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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[brainelectronics]

wonderful