RELEASING.md: Update to tag start of a release's development

At the last cppTango meeting (25/01/2024) we had a discussion about how the TANGO_GIT_REVISION is determined. Currently, git describe on the main branch does not find any of the 9.5.0 tags because we had to resolve merge conflicts when we merged the release/9.5.0 branch back into main. This means that none of the 9.5.0 tags are in the history of main.

In the meeting we decided that we can resolve this by tagging the main branch at the start of the a release cycle with a dev tag (e.g. 10.0.0-dev for the 10.0.0 release).

This MR updates the RELEASING.md to document that process.