Provide link to Contributor License Agreement (CLA)

[goetzk]

Hi, https://github.com/makeplane/docs/blob/master/CONTRIBUTING.md says "Before we can merge your contribution, you must sign our contributor license agreement (CLA)." but doesn't link to one .

I eventually found a reference to it in the original version of https://github.com/makeplane/plane/pull/3642#issuecomment-1940430895. https://cla-assistant.io/makeplane/plane?pullRequest=3642

Could a generic version of this CLA please be linked in contributing.md?

thanks, kk