Document static linking

[ktoso]

@drexin has experience here and can turn this into a guide.

// cc @tomerd

[helje5]

That would be really useful. Not just regular Swift static linking, but also how to link static system libs and such.

[0xTim]

We should also probably document the dependencies required on the system. For instance doing date encoding/decoding on a system that doesn't have tzdata installed causes crashes with no useful stack trace