Update docs for cloning procedure

[mattwr18]

We are now going to recommend developer's clone our repos directly as forks do not have access to the encrypted environment variables in the Travis build.

This means that no PRs from forks will pass the build
see https://docs.travis-ci.com/user/environment-variables/#defining-encrypted-variables-in-travisyml

[roschaefer]

@mattwr18 I have published CYPRESS_TOKEN on our Travis build: https://travis-ci.com/Human-Connection/Human-Connection/builds/106086040#L923. I think it's OK to fork, we should simply encourage new contributors to ask for write permissions instead of forking the repo.

I'll close this for now, feel free to re-open if you think we should keep this issue to update the docs.