README: add links to generator tools

[peterbourgon]

Closes #710, #631, maybe more?

[GrantZheng]

The kujtimiihoxha is not actively working on kit right now. I have forked it, and support go module and fix some problem. The go-kit and kit tool are both great jobs, and deeply used in our team. So I will continue to maintain it in the future. I am very glad to receive any recommend about it.
https://github.com/GrantZheng/kit/releases

[sagikazarmark]

I also have two projects generating Go kit code:

• https://github.com/sagikazarmark/mga
• https://github.com/sagikazarmark/protoc-gen-kit

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@fb88150). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #964   +/-   ##
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  Coverage          ?   74.73%           
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  Files             ?      110           
  Lines             ?     6048           
  Branches          ?        0           
=========================================
  Hits              ?     4520           
  Misses            ?     1314           
  Partials          ?      214

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[peterbourgon]

@sagikazarmark Wow, can we please edit the codecov configuration to turn that ^^ feature off?