Auto gen doc

[songxiaosheng]

What is the purpose of the change

• use code to find all spi extension then gen doc to readme.md

Brief changelog

• add new module dubbo-doc-auto-gen

Verifying this change

you can run then main method of DocAutoGen to gen doc

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a GITHUB_issue field for the change (usually before
  you start working on it). Trivial changes like typos do not require a GITHUB issue. Your pull request should address
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• Format the pull request title like [Dubbo-XXX] Fix UnknownException when host config not exist #XXX. Each commit
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• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency
  exist. If the new feature or significant change is committed, please remember to add sample
  in dubbo samples project.
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[codecov-commenter]

Codecov Report

Merging #224 (499f6de) into master (a6e6eb1) will increase coverage by 0.16%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #224      +/-   ##
============================================
+ Coverage     33.83%   34.00%   +0.16%     
- Complexity      505      507       +2     
============================================
  Files           156      156              
  Lines          5326     5326              
  Branches        662      662              
============================================
+ Hits           1802     1811       +9     
+ Misses         3326     3313      -13     
- Partials        198      202       +4     

see 3 files with indirect coverage changes

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