Added Contribution Checklist

Added Contribution Checklist

Added comprehensive tasklist as a guideline for contributions in How
To Contribute section. Also, minor fixup of API reference :
0_api-introduction.

v2: Added item : Is a backport to a release branch required ?

Bug-AGL: [SPEC-3633]

Signed-off-by: Shankho Boron Ghosh <shankhoghosh123@gmail.com>
Change-Id: I122f2093bd8360be4082497e0cadb67ca67d6d4e
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25580
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
(cherry picked from commit c04ab11b946684902e3e39aef475a2d16c09e1c4)
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25599