Add README

Makefile

@@ -1,8 +1,15 @@
+# C++ compiler executable
 CXX=g++
-CXXFLAGS=-lz
+# C++ compiler flags
+CXXFLAGS=-lz -O2 -static
+# Output file extension (for example .exe for Windows)
+O_EXT=
 
-gd_decr: Main.cpp Base64.hpp GDGameSave.hpp
-	$(CXX) -o $@ $^ $(CXXFLAGS)
+gd_crypt: Main.cpp Base64.hpp GDGameSave.hpp
+	$(CXX) -o $@$(O_EXT) $^ $(CXXFLAGS)
+
+xcomp_linux_win:
+	make CXX=x86_64-w64-mingw32-g++ O_EXT=.exe
 
 clean:
 	rm ./gd_decr

README.md

@@ -0,0 +1,37 @@
+# Geometry Dash Gamesave Cryptor
+## Download
+- The latest Windows binary (.exe) can be downloaded [here](https://github.com/xypwn/gd-gamesave-cryptor/releases)
+
+## Usage
+- `Shift`  + `Right Click` in the folder you dropped the .exe file into
+- Select `Open PowerShell window here`
+- In the PowerShell window, type one of the following:
+	- `.\gd_crypt -d` to decrypt and store the result in the current folder
+	- `.\gd_crypt -e` to encrypt and store the result in the current folder
+	- `.\gd_crypt -es` to encrypt and store the result in the Geometry Dash folder
+
+## Building from source on Linux
+#### Building for Linux
+- If you are using Proton to play GD, do not use Wine for the Windows version. Use this version instead as it automatically uses the correct path for Proton.
+- Clone the repository and cd into it
+
+`$ git clone https://github.com/xypwn/gd-gamesave-cryptor.git`
+
+`$ cd gd-gamesave-cryptor`
+- Build the binary:
+
+`$ make`
+- The executable is named `gd_crypt` and can be run with `./gd_crypt`
+#### Building for Windows
+- You will need to install your distro's version of `mingw-w64`
+	- `sudo apt install mingw-w64` on Ubuntu
+	- Install the AUR package `mingw-w64-gcc` on Arch Linux
+- Clone the repository and cd into it
+
+`$ git clone https://github.com/xypwn/gd-gamesave-cryptor.git`
+
+`$ cd gd-gamesave-cryptor`
+- Build the Windows binary:
+
+`$ make xcomp_linux_win`
+-  The executable is named `gd_crypt.exe`