README: don't use github features for contributions
README: don't use github features for contributions

Display the warning ASAP to help not wasting time for newcomers. All kind of
contributions are expected at the mailing lists to bring posts a wider audience.

Signed-off-by: Nicolas Sebrecht <347709267ddd1958560c4c317ea3fd613f10a719@laposte.net>

frioux/offlineimap,frioux/offlineimap