Improve README.md.

- Add Arch Linux installation instructions.
- Add Mac OS X installation instructions.
- Add screencast.
- Add Vim usage instructions.
- Add instructions for checking the signature.

Closes #34.

README.md

@@ -1,24 +1,87 @@
 # Pick
 
+![pick(1) usage](screencast.gif)
+
 The `pick(1)` utility allows users to choose one option from a set of choices
 using an interface with fuzzy search functionality.
 
 ## Installation
 
-1. Download the latest release from [the releases page] and extract the archive:
+### Arch Linux
+
+A package for Pick is available in [the AUR].
+
+```sh
+wget https://aur.archlinux.org/packages/pi/pick/pick.tar.gz
+tar -xzf pick.tar.gz
+cd pick
+makepkg -s
+pacman -U pick-VERSION-x86_64.pkg.tar.xz
+```
+
+### Mac OS X via Homebrew
+
+```sh
+brew tap thoughtbot/formulae
+brew install pick
+```
+
+### From source
+
+1. Download the latest release from [the releases page], optionally check the
+   signature and extract the archive:
 
-        tar -zxvf pick-VERSION.tar.gz
+```sh
+wget https://github.com/thoughtbot/pick/releases/download/vVERSION/pick-VERSION.tar.gz
+wget https://github.com/thoughtbot/pick/releases/download/vVERSION/pick-VERSION.tar.gz.asc
+gpg --verify pick-VERSION.tar.gz.asc
+tar -xzf pick-VERSION.tar.gz
+```
 
 2. Follow the bundled installation instructions:
 
-        cd pick-VERSION
-        less INSTALL.md
+```sh
+cd pick-VERSION
+less INSTALL.md
+```
 
-[the releases page]: https://github.com/thoughtbot/pick/releases
+[the AUR]: https://aur.archlinux.org/packages/pick/
+[the releases page]: https://github.com/thoughtbot/pick/releases/
 
 ## Usage
 
-See the `pick(1)` man page for usage instructions and examples.
+`pick(1)` reads a list of choices on `stdin` and outputs the selected choice on
+`stdout`. Therefore it is easily used both in pipelines and subshells:
+
+```sh
+git ls-files | pick | xargs less # Select a file in the current git repository to view in less
+cd $(find . -type d | pick)      # Select a directory to cd into
+```
+
+Pick can also easily be used from within Vim with this VimL function:
+
+```viml
+function! PickCommand(choice_command, pick_args, vim_command)
+  try
+    let selection = system(a:choice_command . " | ./src/pick" . a:pick_args)
+  catch /Vim:Interrupt/
+    " Swallow the ^C so that the redraw below happens; otherwise there will be
+    " leftovers from pick on the screen
+    redraw!
+    return
+  endtry
+  redraw!
+  exec a:vim_command . " " . selection
+endfunction
+```
+
+Then map keys to call this function:
+
+```viml
+nnoremap <leader>e :call PickCommand("git ls-files", "", ":e")<cr>
+```
+
+See the `pick(1)` man page for detailed usage instructions and more examples.
 
 ## Copyright