[docs] Clean the M2 repository before compiling the release version.
[docs] Clean the M2 repository before compiling the release version.

In the RELEASE_README.md file, the command line for cleaning the M2
repository is added before the ones for compiling.
Cleaning the M2 repository before compiling makes the process safer and
almost common to a large amount of developers.

Signed-off-by: Stéphane Galland <23860b554af9116d4237121086bd6767cdd4ff4b@arakhne.org>

gallandarakhneorg/sarl,gallandarakhneorg/sarl,sarl/sarl,sarl/sarl,gallandarakhneorg/sarl,sarl/sarl,jgfoster/sarl,jgfoster/sarl,jgfoster/sarl,sarl/sarl,sarl/sarl,sarl/sarl,jgfoster/sarl