docs: update README

[ycexiao]

What problem does this PR address?

Closes #13

Update README and add diffpy.apps entry point according to the README.

What should the reviewer(s) do?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (a06f7f0) to head (0a48d18).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   92.46%   92.94%   +0.48%     
==========================================
  Files           4        4              
  Lines         146      156      +10     
==========================================
+ Hits          135      145      +10     
  Misses         11       11              

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[ycexiao]

@sbillinge, it's ready for review.

[ycexiao]

The package name(diffpy.apps) and the CLI app(diffpy.app) name are easy to confuse. We use diffpy.app because each time only one app is invoked.

Also add diffpy.apps as an entry point for user's convenience when they didn't notice the difference between the package name and the entry point.