README: Recommend sourcing the bootstrapper

Thanks to @svnpenn and @nvartolomei in https://github.com/mathiasbynens/dotfiles/commit/a77f4872f26d681a35cface9d6bcb4ad55fec752#commitcomment-2100022, and thanks to @janmoesen for the BASH_SOURCE pointer.

README.md

@@ -7,19 +7,19 @@
 You can clone the repository wherever you want. (I like to keep it in `~/Projects/dotfiles`, with `~/dotfiles` as a symlink.) The bootstrapper script will pull in the latest version and copy the files to your home folder.
 
 ```bash
-git clone https://github.com/mathiasbynens/dotfiles.git && cd dotfiles && ./bootstrap.sh
+git clone https://github.com/mathiasbynens/dotfiles.git && cd dotfiles && source bootstrap.sh
 ```
 
 To update, `cd` into your local `dotfiles` repository and then:
 
 ```bash
-./bootstrap.sh
+source bootstrap.sh
 ```
 
 Alternatively, to update while avoiding the confirmation prompt:
 
 ```bash
-./bootstrap.sh -f
+set -- -f; source bootstrap.sh
 ```
 
 ### Git-free install

bootstrap.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-cd "$(dirname "$0")"
+cd "$(dirname "${BASH_SOURCE}")"
 git pull
 function doIt() {
 	rsync --exclude ".git/" --exclude ".DS_Store" --exclude "bootstrap.sh" --exclude "README.md" -av . ~