document how to install the completion files

[daxim]

The release archive ripgrep-0.3.1-x86_64-unknown-linux-musl.tar.gz contains a number of completion files, but no instructions how to use them AFAICS.

[BurntSushi]

I would love to provide documentation, but I don't know how to use them myself. I could figure out bash, but I'm unlikely to figure out the others.

[kbknapp]

rustup has some simple documentation.

[daxim]

Thank you, I used that info in order to write the following documentation patch.

Shell completions
=================

Bash
----

Move `rg.bash-completion` to [`$XDG_CONFIG_HOME/bash_completion`](https://github.com/scop/bash-completion/blob/master/doc/bash_completion.txt#L4) or `/etc/bash_completion.d/`.

Fish
----

Move `rg.fish` to `$HOME/.config/fish/completions`.

Apply it where you see fit.