Add inline documentation for {start, exceptions}.S, and update README.

This PR adds inline documentation for key places in start.S and exceptions.S. It explains how `arm_fault_frame` is constructed, the role of the `saveall_offset` function, and the tricks when pushing registers on the stack while presenting the current status of the stack after each call. Additionally, it specifies where the functions defined, helping the reader make connections easily without getting lost in the source.

In README, it presents how to build LK. In this file, I've presented my understanding on how build system works depending on the conversation with Travis on IRC. I did not delve into that much.

I would appreciate if you review the comments. I tried to be clear as clear as possible for a reader who has an interest in LK. If there are any mistakes, they are welcomed.

Regards,
Eren