update README

README.md

 # NEMU
 
-NEMU(NJU Emulator) is a simple but complete full-system x86 emulator designed for teaching purpose.
-Small x86 programs can run under NEMU.
+NEMU(NJU Emulator) is a simple but complete full-system emulator designed for teaching purpose.
+Currently it supports x86, mips32, and riscv32.
+To build programs run above NEMU, refer to the [AM project](https://github.com/NJU-ProjectN/nexus-am.git).
 
 The main features of NEMU include
 * a small monitor with a simple debugger
@@ -10,17 +11,23 @@ The main features of NEMU include
   * expression evaluation without the support of symbols
   * watch point
   * differential testing with reference design (e.g. QEMU)
-* CPU core with support of most common used x86 instructions in protected mode
-  * real mode is not supported
-  * x87 floating point instructions are not supported
+  * snapshot
+* CPU core with support of most common used instructions
+  * x86
+    * real mode is not supported
+    * x87 floating point instructions are not supported
+  * mips32
+    * CP1 floating point instructions are not supported
+  * riscv32
+    * only RV32IM
 * memory
-* I386 paging
-  * TLB is optional
+* paging
+  * TLB is optional (but necessary for mips32)
   * protection is not supported
-* I386 interrupt and exception
+* interrupt and exception
   * protection is not supported
-* 4 devices
-  * serial, timer, keyboard, VGA
+* 5 devices
+  * Args Rom, serial, timer, keyboard, VGA
   * most of them are simplified and unprogrammable
 * 2 types of I/O
   * port-mapped I/O and memory-mapped I/O