[DOC] Release notes should mention when a release is not published on CRAN

[tbouffard]

NOTE: this will apply after the first CRAN publishing, see #10

The CRAN policies state (see https://r-pkgs.org/release.html#cran-policies and https://cran.r-project.org/web/packages/policies.html)

Submitting updates should be done responsibly and with respect for the volunteers’ time. Once a package is established (which may take several rounds), “no more than every 1–2 months” seems appropriate.

However, on our side, we can provide more frequent releases (think about what we do with the bpmn-visualization TypeScript library every 2 weeks at the minimum). In this case, they are only available by installing from GitHub and this is fine.
In this case, the Release Notes should mention that to warn users.

To not forget it, the documentation of the release process should be updated. The release notes should contain a sentence explaining why it is not published on CRAN