Add explanation of submodule usage in README.md

[dongxuny]

No description provided.

[codecov-commenter]

Codecov Report

Merging #38 (85bb1b6) into master (878e7ee) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   85.45%   85.45%           
=======================================
  Files           1        1           
  Lines          55       55           
=======================================
  Hits           47       47           
  Misses          5        5           
  Partials        3        3           

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