Document installation

[winebarrel]

Add an Installation section to the README, just above Usage.

• Download an archive from the releases page and put azpim on PATH;
  filenames follow GoReleaser's default naming, and checksums.txt is
  mentioned for anyone who wants to verify one.
• go install github.com/winebarrel/azpim/cmd/azpim@latest, with the Go
  version taken from go.mod.

--version works either way: resolveVersion falls back to the embedded
build info when GoReleaser hasn't stamped a version in.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.57%. Comparing base (415b0fa) to head (dbd07f9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #6   +/-   ##
=======================================
  Coverage   96.57%   96.57%           
=======================================
  Files           7        7           
  Lines         496      496           
=======================================
  Hits          479      479           
  Misses         17       17           

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