add README

README.md

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+# Building
+
+To run the pre-compiled version on Arch Linux, you need some basic libs:
+
+```
+pacman -S glibc libcap gcc-libs alsa-lib systemd-libs
+```
+
+To build the game on Arch Linux yourself, you need these, although you can replace `rust` with `rustup`:
+
+```
+pacman -S rust libx11 pkgconf alsa-lib
+```
+
+Then run the following commands, replacing `[URL]` with the clone URL of the git repository:
+
+```
+git clone [URL]
+cd outfly
+cargo run
+```