Update README.md

[ytdHuang]

Summary of this PR:

• Change the download badge (from counting monthly downloads to counting all-time downloads)
• Update Installation for QuantumToolbox.jl

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.04%. Comparing base (9b1f668) to head (63b8e5f).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #130   +/-   ##
=======================================
  Coverage   91.04%   91.04%           
=======================================
  Files          21       21           
  Lines        1765     1765           
=======================================
  Hits         1607     1607           
  Misses        158      158           

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