docs: zephyr: add "Installing Requirements and Dependencies"
docs: zephyr: add "Installing Requirements and Dependencies"

This patch documents requirements and dependencies necessary to build
mcuboot on Zephyr.

Signed-off-by: Piotr Mienkowski <be7d9ab625d3cb5e3a4a3202f3529b954dba084c@gmail.com>

runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,utzig/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot,runtimeco/mcuboot