[CONTRIBUTING] Add note about only using BusyBox sh in hooks.
[CONTRIBUTING] Add note about only using BusyBox sh in hooks.

Until the Supervisor is made more intelligent about which shell
interpreter to execute, all hooks written for core Plans must assume a
minmal bourne shell, such as BusyBox's `sh`, and never assume GNU
Bashims. This will cause breakages when running on systems which have
`/bin/sh` using a dash or non GNU Bash mode.

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

be-plans/be,be-plans/be,be-plans/be,be-plans/be