Merge branch 'master' into yummin
Change process_execute() in process.c

1. Modify README
2. Add DESIGNDOC for Project 2
Implement some functions in process.c, syscallc.c
Simplify the implementation of process_execute
Implement functions in process.c

Implement process_wait and process_exit, rewrite start_process.
Passed some tests.
Update code: Implement some functions
Fix typo and add functions
Merge remote-tracking branch 'origin/yummin' into borui

Conflicts:
	src/userprog/syscall.c
Merge cont. - reformat