Add Arch install process

INSTALL.md

@@ -28,6 +28,19 @@ cd hush3
 ./build.sh -j3
 ```
 
+## Build on Arch:
+
+```sh
+# install build dependencies
+sudo pacman -S gcc libsodium lib32-zlib unzip wget git python rust curl autoconf cmake
+# clone git repo
+git clone https://git.hush.is/hush/hush3
+cd hush3
+# Build
+# This uses 3 build processes, you need 2GB of RAM for each. 
+./build.sh -j3
+```
+
 ## Build on Fedora:
 
 ```sh