add Mailing list and subscribe method

[lovepoem]

Ⅰ. Describe what this PR did

add Mailing list and subscribe method in the README.md

Ⅱ. Does this pull request fix one issue?

yes

[slievrly]

LGTM

[codecov-io]

Codecov Report

Merging #666 into develop will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #666   +/-   ##
==========================================
  Coverage      32.24%   32.24%           
  Complexity       877      877           
==========================================
  Files            227      227           
  Lines           8784     8784           
  Branches        1058     1058           
==========================================
  Hits            2832     2832           
  Misses          5630     5630           
  Partials         322      322

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