Ftr: add tools introduce in readme for 3.0

[cjphaha]

add introduction to the dubbo-go command line tool

[codecov-commenter]

Codecov Report

Merging #1714 (2f6b50b) into 3.0 (20e6451) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 2f6b50b differs from pull request most recent head 4ac09a8. Consider uploading reports for the commit 4ac09a8 to get more accurate results

@@            Coverage Diff             @@
##              3.0    #1714      +/-   ##
==========================================
+ Coverage   41.29%   41.31%   +0.02%     
==========================================
  Files         255      255              
  Lines       14705    14705              
==========================================
+ Hits         6073     6076       +3     
+ Misses       7933     7931       -2     
+ Partials      699      698       -1     

Impacted Files	Coverage Δ
filter/metrics/filter.go	100.00% <0.00%> (+15.00%)	⬆️

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