Add initial instructions about Arch Linux

Author: ktf

building/custom.md

@@ -56,6 +56,7 @@ packages to install and configurations to perform.
     * Specify the corresponding Ubuntu architecture when running the `aliBuild` command
       using the `-a` option (e.g. `-a ubuntu2004_x86-64` for Ubuntu 20.04).
       Use the `-a` option also with the `alienv` command.
+* [Arch Linux (Preliminary)](prereq-arch.md)
 
 If your operating system is _not_ in any list, it does not mean our software won't work on it;
 it will be just more difficult for you to get support for it.

building/prereq-arch.md

@@ -0,0 +1,17 @@
+aliBuild prerequisites for ArchLinux
+=================================
+
+ALICE software on ArchLinux is supported on a best effort basis. There is no guarantee that software builds or runs correctly. Support requests might have low priority.
+
+## Install or Upgrade the Required Packages
+
+With root permissions, i.e. `sudo` or as `root` install the prerequisites using:
+
+```bash
+pacman -S git python3 python-pip which gcc \
+          make gcc-fortran \
+          base-level libxpm libxft glu gsl \
+          libuv tbb xerces-c re2 libwebsockets grpc
+```
+
+You are now ready to [start building ALICE software](README.md#get-or-upgrade-alibuild)