add docs for sending voice message & generating voice file

[wj-Mcat]

• [x] convert voice(silk) to mp3 file
• [x] convert mp3 to voice(silk) file
• [ ] create dockerfile to voice service which is more friendly for developer

[codecov[bot]]

Codecov Report

Merging #329 (d063f07) into master (6940577) will increase coverage by 0.49%. The diff coverage is 57.14%.

:exclamation: Current head d063f07 differs from pull request most recent head d18411b. Consider uploading reports for the commit d18411b to get more accurate results

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   40.49%   40.98%   +0.49%     
==========================================
  Files          27       27              
  Lines        1936     1942       +6     
==========================================
+ Hits          784      796      +12     
+ Misses       1152     1146       -6     

Flag	Coverage Δ
unittests	40.98% <57.14%> (+0.49%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/wechaty/plugin.py	28.36% <57.14%> (+0.48%)	:arrow_up:
src/wechaty/utils/async_helper.py	82.35% <0.00%> (+26.47%)	:arrow_up: