1_agl-compositor.md: Added initial docs for agl-compositor

1_agl-compositor.md: Added initial docs for agl-compositor

Contains some project history, and some Wayland introduction.  Goes a
bit explaining the private extensions, the toolkit platforms supported
and some explanation of policy engines and policies. Some
particularities to the agl-compositor been described as well.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I444144dba024643c0fe639bff2a11ca8f77df8c4
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25958
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>