Add installation instructions

[djhoese]

Copies the installation instructions from Satpy and modifies them slightly.

• Closes Add install instructions #72 (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[codecov]

Codecov Report

Merging #105 (4a6be55) into main (baae46c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #105   +/-   ##
=======================================
  Coverage   92.51%   92.51%           
=======================================
  Files          10       10           
  Lines        3432     3432           
=======================================
  Hits         3175     3175           
  Misses        257      257           

Flag	Coverage Δ
unittests	92.51% <ø> (ø)

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