Add CONTRIBUTING.md

[yahavi]

• All tests passed. If this feature is not already covered by the tests, I added new tests.
• All static analysis checks passed.
• This pull request is on the dev branch.
• I used gofmt for formatting the code before submitting the pull request.

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Add a CONTRIBUTING.md file and reformat the pull request template.
Goals:

• Make the README.md file clean from technical details.
• Help contributors by pointing them directly to the CONTRIBUTING.md file. New contributed see a link to it once opening a Pull Request:
  
• Replace an unactionable guideline "All static analysis checks passed" in the pull request template with a requirement to run go vet ./....

Link to README: https://github.com/yahavi/jfrog-cli/tree/contributing#readme