Update readme to standard thoughtbot readme.

README.md

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+thoughtbot dotfiles
+===================
+
+Install
+-------
+
+First, [fork this repo](https://github.com/thoughtbot/dotfiles#fork_box) on Github.
+
+Then, clone your Github fork (replace "your-github-name" with your Github name) onto your laptop and install it:
+
+    git clone git@github.com:your-github-name/dotfiles.git
+    cd dotfiles
+    ./install.sh
+
+This will create symlinks for all config files in your home directory. You can
+safely run this file multiple times to update.
+
+There is configuration for `zsh` so switch your shell from the default `bash` to `zsh` on OS X:
+
+    chsh -s /bin/zsh
+
+Why fork?
+---------
+
+dotfiles are fairly personal. You should be able to modify your dotfiles, and save them in version control in your fork.
+
+However, the thoughtbot folks are often tweaking these dotfiles and you want to be able to get those updates.
+
+So, your master branch is meant for your customizations and use the `upstream` branch to get thoughtbot's updates.
+
+Set up the upstream branch
+--------------------------
+
+You only have to do this once:
+
+    git remote add upstream git@github.com:thoughtbot/dotfiles.git
+    git fetch upstream
+    git checkout -b upstream upstream/master
+
+Update thoughtbot's changes into your customizations
+----------------------------------------------------
+
+You will want to customize your environment. We suggest making changes in files that are not in thoughtbot's files.
+
+For example, to customize your `zsh` config, make your changes in `~/.zshenv`:
+
+    # RVM
+    [[ -s '/Users/croaky/.rvm/scripts/rvm' ]] && source '/Users/croaky/.rvm/scripts/rvm'
+
+    # recommended by brew doctor
+    export PATH="/usr/local/bin:/usr/local/sbin:$PATH"
+
+Commit those kinds of things in your master branch.
+
+Then, each time you want to update thoughtbot's changes.
+
+    git checkout upstream
+    git pull
+    git checkout master
+    git rebase upstream
+