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

@@ -22,11 +22,10 @@ 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
-should appear when Doom is loaded and executed.
+The build script will then download data file for Doom automatically. SDL2 based window should appear when Doom is loaded and executed. The usage and limitations of Doom and Quake demo are listed in [docs/demo.md](docs/demo.md).
 
 ## riscv-arch-test
 

build/quake.elf

@@ -0,0 +1,48 @@
+# Demos
+
+## Doom
+**source**: [doom_riscv](https://github.com/smunaut/doom_riscv)
+
+**commmad**: `make doom`
+
+[Doom](https://en.wikipedia.org/wiki/Doom_(franchise)), a pioneering first-person shooter game developed by [id Software](https://en.wikipedia.org/wiki/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/bLc5LG8.gif)
+
+### Limitations
+* The input system is not supported yet
+
+## Quake
+**source**: [quake-embedded](https://github.com/sysprog21/quake-embedded/)
+
+**command**: `make quake`
+
+[Quake](https://en.wikipedia.org/wiki/Quake_(series)) 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/gXKb7D0.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