add build instructions for archlinux users

[vimkim]

I use arch btw.

sudo pacman -S capnproto python-pexpect lldb

The above 3 were the packages that I needed to install to build rr on arch linux.

Thanks for creating this amazing project.

[rocallahan]

You can go ahead and edit the wiki yourself.

[GitMensch]

@vimkim That would mean adjusting https://github.com/rr-debugger/rr/wiki/Building-And-Installing#dependencies-installation - note that ideally you start with a fresh arch environment (not something that is already setup to build a bunch of other software).