Update demo executable along with manual

This commit updates Quake ELF executable file and adds docs/demo.md
which describes the usage and limitations of demo games, the rule name
of Doom demo under Makefile is changed to "doom" instead of "demo".

Author: alanjian85

Makefile

@@ -103,7 +103,7 @@ include mk/external.mk
 
 # Non-trivial demonstration programs
 ifeq ("$(ENABLE_SDL)", "1")
-demo: $(BIN) $(DOOM_DATA)
+doom: $(BIN) $(DOOM_DATA)
 	(cd $(OUT); ../$(BIN) doom.elf)
 ifeq ("$(ENABLE_RV32F)", "1")
 quake: $(BIN) $(QUAKE_DATA)

README.md

@@ -22,7 +22,7 @@ make check
 
 Run [Doom](https://en.wikipedia.org/wiki/Doom_(1993_video_game)), the classical video game, via `rv32emu`:
 ```shell
-make demo
+make doom
 ```
 
 The build script will then download data file for Doom automatically. SDL2 based window

build/quake.elf

@@ -0,0 +1,48 @@
+# Demos
+
+## Doom
+**source**: [doom_riscv](https://github.com/smunaut/doom_riscv)
+
+**commmad**: `make doom`
+
+Doom, a pioneering first-person shooter game developed by id Software in 1993, is known for its open-source code and vibrant community, it debuted innovations like genuine 3D graphics, networked multiplayer gameplay and the ability for players to create custom expansions. 
+
+![Doom Gameplay](https://imgur.com/gXKb7D0.gif)
+
+### Limitations
+* The input system is not supported yet
+
+## Quake
+**source**: [quake-embedded](https://github.com/sysprog21/quake-embedded/)
+
+**command**: `make quake`
+
+Quake was created in 1996 as a successor of the highly succcessful first-person shooter game Doom, it is based on Doom's game engine and 3D graphics compability, and it enhanced the fast-paced gameplay and online multiplayer over the Internet. 
+
+[quake-embedded](https://github.com/sysprog21/quake-embedded/) is an embedded RISC-V port of Quake based on [QuakEMBD](https://github.com/FantomJAC/quakembd).
+
+Build Instruction:
+```
+$ git clone https://github.com/sysprog21/quake-embedded.git && cd quake-embedded
+$ mkdir build && cd build
+$ cmake -DCMAKE_TOOLCHAIN_FILE=../port/boards/rv32emu/toolchain.cmake \
+        -DCMAKE_BUILD_TYPE=RELEASE -DBOARD_NAME=rv32emu ..
+$ make
+```
+
+![Quake Gameplay](https://imgur.com/bLc5LG8.gif)
+
+### Default Key Bindings
+* Move Forward/Backward: Up Arrow Key/Down Arrow Key
+* Move Left/Right: Comma(,) Key/Period(.) Key
+* Turn Left/Right: Left Arrow Key/Right Arrow Key
+* Swim Up/Down: D Key/C Key
+* Shoot: Left Mouse Button or CTRL Key
+* Switch Weapon: Slash(/) Key
+* Sprint: Shift Key
+
+You may use the mouse to adjust the pitch and yaw angle
+
+### Limitations
+* Mouse wheel input is not supported
+* All sound functions are not implemented