Update compilation steps

[WeiLoy]

No description provided.

[benbaley]

according to releasenotes of golang_v1.16 go mod is not auto download
please add comment

[benbaley]

missing 'cd PlatON-Go' ?

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (feature-mainnet-launch@958122f). Click here to learn what that means.
The diff coverage is n/a.

@@                    Coverage Diff                    @@
##             feature-mainnet-launch    #1740   +/-   ##
=========================================================
  Coverage                          ?   50.27%           
=========================================================
  Files                             ?      462           
  Lines                             ?    56556           
  Branches                          ?        0           
=========================================================
  Hits                              ?    28435           
  Misses                            ?    24622           
  Partials                          ?     3499           

Flag	Coverage Δ
unittests	50.27% <0.00%> (?)

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