docs: add instructions to use diffpy.apps runmacro <macro-file>

[ycexiao]

What problem does this PR address?

Closes #4

What should the reviewer(s) do?

[ycexiao]

@sbillinge, it's ready for review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.46%. Comparing base (c0de591) to head (a06f7f0).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #10   +/-   ##
=======================================
  Coverage   92.46%   92.46%           
=======================================
  Files           4        4           
  Lines         146      146           
=======================================
  Hits          135      135           
  Misses         11       11           

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