Added RBA tool documentation

Added RBA tool documentation

Syntax of .rba file

Bug-AGL: SPEC-3947

Signed-off-by: Anusha Gugale <external.agogale@jp.adit-jv.com>
Change-Id: Ibac6b1a38a1ed2a149cf4e45fad180d427d8e174
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/26541
Reviewed-by: Kenji Hosokawa <khosokawa@jp.adit-jv.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Naohiro Nishiguchi <nnishiguchi@jp.adit-jv.com>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>