SiFive-Core Version 0.0.2

This update includes:

Added

• Explanation of current state of PAUSE instruction
• Clearify that feature disable must run on M mode
• Support the NMI registers and instructions

Modified

• Set CEASE to be unsafe function
• Rearrange documents
• Temporarily hide SiFive platform features

Removed

• PAUSE instruction, it will be in core::hint::spin_loop