docs: Document building qemuarm64 with openharmony support

Until !250 (merged) has landed, the wic images don't boot on qemuarm64.

Document what needs to be done to get a booting image for openharmony integration testing.

Signed-off-by: Bernhard Rosenkränzer bernhard.rosenkraenzer.ext@huawei.com

Closes eclipse/oniro-core/meta-openharmony#57