Add OTA documentation

Add documentation and notes on the OTA stack.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>
Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Kareem Zarka <kareem.zarka@huawei.com>