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

doc/rg.1

@@ -409,3 +409,14 @@ ripgrep.
 Note that this must be passed to every invocation of rg.
 .RS
 .RE
+.SH SHELL COMPLETION
+.PP
+Shell completion files are included in the release tarball for Bash,
+Fish, Zsh and PowerShell.
+.PP
+For \f[B]bash\f[], move \f[C]rg.bash\-completion\f[] to
+\f[C]$XDG_CONFIG_HOME/bash_completion\f[] or
+\f[C]/etc/bash_completion.d/\f[].
+.PP
+For \f[B]fish\f[], move \f[C]rg.fish\f[] to
+\f[C]$HOME/.config/fish/completions\f[].