Add docs for shell completion files.

Add small howtos for installing shell completion files to the README and
the man page.

They are still incomplete. We're missing Zsh and PowerShell.

Fixes #262

README.md

@@ -302,6 +302,16 @@ extensions.
 The syntax supported is
 [documented as part of Rust's regex library](https://doc.rust-lang.org/regex/regex/index.html#syntax).
 
+### Shell completions
+
+Shell completion files are included in the release tarball for Bash, Fish, Zsh
+and PowerShell.
+
+For **bash**, move `rg.bash-completion` to `$XDG_CONFIG_HOME/bash_completion`
+or `/etc/bash_completion.d/`.
+
+For **fish**, move `rg.fish` to `$HOME/.config/fish/completions`.
+
 ### Building
 
 `ripgrep` is written in Rust, so you'll need to grab a