[LAUNCH DOCS] Add instructions what needs to be installed, and how to README

[mikekgfb]

At present, running the instructions in the README will fail for the xcode project. See #755

At a minimum we should specify what should be installed and what the minimum xcode version (and any other requirements) are?

Also, I would expect this to fail even then, because like this might be GUI based with no fully scriptable set of instructions (plus it's not clear we'd want the script instructions when most devs are more likely going to like to start around with the GUI builder?). So, how can/should we test iOS app build in open source?

As a corollary, how do we automate testing of README for correctness? (and maybe the answer is "it's too involved", and that's OK if that turns out to be the right answer)

cc: @byjlw @shoumikhin

[shoumikhin]

@mikekgfb is this issue to flag that the current readme instructions don't work, or that they need to be expanded with more details, or if we can automate those instructions and run on CI?