Add supported platform to README

[funkyboy]

Fix #692

[paddybyers]

Typo: The following platforms.

So if someone builds an app embedding this library will that app fail to install on iOS8? Will it run but possibly fail at runtime?

I think a statement about platform support needs to be explicit about what it means for a platform to be unsupported.

[funkyboy]

@paddybyers Technically I didn't drop the compatibility with iOS8 yet in the configuration. So it will "work", as in compile and not throw runtime errors.

I am afraid the message in the README can be quite long:

• we are not breaking compatibility with iOS8 on purpose
• but we are not running tests on iOS8
• so we don't know how it behaves on that platform
• and if you spot a bug specific to iOS8 it's likely that we are not going to fix it, or give it low priority.

I'd keep the positive version: "we support x, y and z". Or even "this SDK is compatible with iOS9 and above".
Firebase for example says "An Xcode project targeting iOS 8 or above".

[paddybyers]

I'd keep the positive version: "we support x, y and z". Or even "this SDK is compatible with iOS9 and above".

OK, fine. If we later we or customers find things that are broken on earlier versions we can decide whether we explicitly state that it is not compatible with iOS 8, or we have a statement that says something like "some features are not available on earlier iOS versions; see <issue>".

[funkyboy]

@paddybyers Improved again the description.

[paddybyers]

LGTM, thanks