Add DCO requirements and updated code of conduct

[willtsai]

Issue reference: dapr/docs#2039

Description

Add DCO requirements and updated code of conduct. Content inspired and borrowed from https://github.com/kedacore/keda/blob/main/CONTRIBUTING.md#developer-certificate-of-origin-signing-your-work

Issue reference

dapr/docs#2039

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]
• Specification has been updated / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]
• Provided sample for the feature / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]

[codecov]

Codecov Report

Merging #669 (a383102) into master (413a036) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #669   +/-   ##
=========================================
  Coverage     77.25%   77.25%           
  Complexity      946      946           
=========================================
  Files            88       88           
  Lines          3012     3012           
  Branches        334      334           
=========================================
  Hits           2327     2327           
  Misses          523      523           
  Partials        162      162           

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