CONTRIBUTING: add more hints on what is required to cut a release

[xrmx]

What does this pull request do?

Warn that tags are signed and you need to have a pgp key and to enable ssh keys for SSO.

Related issues

None

[xrmx]

One also needs to have some cautions when cloning:

ERROR: The 'elastic' organization has enabled or enforced SAML SSO.
To access this repository, you must use the HTTPS remote with a personal access token or SSH with an SSH key and passphrase that has been authorized for this organization.
Visit https://docs.github.com/articles/authenticating-to-a-github-organization-with-saml-single-sign-on/ for more information.

[xrmx]

Cannot push to 6.x branch because it's protected:

$ git push upstream 6.x
0 oggetti totali (0 delta), 0 riutilizzati (0 delta), 0 riutilizzati nel file pack
remote: error: GH006: Protected branch update failed for refs/heads/6.x.
remote: error: 3 of 4 required status checks have not succeeded: 2 expected and 1 failing.
To github.com:elastic/apm-agent-python.git
 ! [remote rejected]   6.x -> 6.x (protected branch hook declined)

[xrmx]

Updated after the discussion