Update zsh instructions and add bash instructions

etc/README.md

 # Installation instructions
 
+## bash
+
+Open your `.bashrc` file and add:
+
+> If you want to set up Git to automatically have Bash shell completion for all users, copy the `hub.bash_completion` script to the `/opt/local/etc/bash_completion.d` directory on Mac systems or to the `/etc/bash_completion.d/` directory on Linux systems. ([Source](https://git-scm.com/book/en/v1/Git-Basics-Tips-and-Tricks#Auto-Completion))
+
+* [Link to git auto-completion bash file](https://github.com/git/git/blob/master/contrib/completion/git-completion.bash)
+
+```sh
+# Make sure you've aliased hub to git
+eval "$(hub alias -s)"
+
+# And make sure that the git auto-completion is being loaded
+if [ -f /path/to/git-completion.bash ]; then
+    . /path/to/git-completion.bash
+fi
+
+# Load hub autocompletion
+if [ -f /path/to/hub.bash_completion ]; then
+    . /path/to/hub.bash_completion
+fi
+```
+
 ## zsh
 
 Create a new folder for completions:
@@ -8,15 +31,16 @@ Create a new folder for completions:
 mkdir -p ~/.zsh/completions
 ```
 
-Download the file hub.zsh_completion and rename it to `_hub`:
+Copy the file `/etc/hub.zsh_completion` from the location where you downloaded `hub` to the folder `~/.zsh/completions/` and rename it to `_hub`:
 
 ```sh
-curl https://github.com/github/hub/blob/master/etc/hub.zsh_completion >  ~/.zsh/completions/_hub
+cp /path/to/etc/hub.zsh_completion ~/.zsh/completions/ \
+    mv ~/.zsh/completions/hub.zsh_completion ~/.zsh/completions/_hub
 ```
 
-Then add the following lines to your .zshrc file:
+Then add the following lines to your `.zshrc` file:
 
 ```sh
 fpath=(~/.zsh/completions $fpath) 
 autoload -U compinit && compinit
-```
\ No newline at end of file
+```