manual: contributing: expand introduction

This patch expands the introduction of the "Contributing to buildroot"
section. In general, the intention of this and subsequent patches is to
promote non-feature contributions, like autobuild fixes, bug report
analysis, TODO list work, etc.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>