docs(contribution): add CLA section

[yoshinorin]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

Motivation / Use-Case

Current contribution guide does not exist about CLA. IMHO It is better to have CLA for a contributor.

I wrote it referencing webpack-cli.

[codecov]

Codecov Report

Merging #1963 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1963   +/-   ##
=======================================
  Coverage   92.55%   92.55%           
=======================================
  Files          28       28           
  Lines        1075     1075           
  Branches      321      321           
=======================================
  Hits          995      995           
  Misses         76       76           
  Partials        4        4

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[hiroppy]

Thanks