Update README.md

Author: wyfcyx

README.md

@@ -32,12 +32,13 @@ Now we are still updating our project, you can find latest changes on branches `
 * [x] fix `power` series application in early chapters, now you can find modulus in the output
 * [x] use `UPSafeCell` instead of `RefCell` or `spin::Mutex` in order to access static data structures(now ch4 completed)
 * [x] move `TaskContext` into `TaskControlBlock` instead of restoring it in place on kernel stack(since ch3), eliminating annoying `task_cx_ptr2`
+* [x] replace `llvm_asm!` with `asm!`(now ch4 completed)
 
 ### Todo(High priority)
 
-* [ ] replace `llvm_asm!` with `asm!`
+* [ ] adjust API of `UPSafeCell` so that it cannot be borrowed twice at a time
 * [ ] bug fix: we should call `find_pte` rather than `find_pte_create` in `PageTable::unmap`
-* [ ] add a new chapter about synchronization & mutual exclusion(up or smp?)
+* [ ] add a new chapter about synchronization & mutual exclusion(uniprocessor only)
 
 ### Todo(Low priority)