[README] Fix HTML for MacOS and Linux instructions

Readme.md was not displaying lists and links correctly under <details/>.  I fixed this by adding newlines between the <details/> tags and the interior content. 

Credit to user [SCMX](https://gist.github.com/scmx/eca72d44afee0113ceb0349dd54a84a2#code).

Readme.md

@@ -111,6 +111,7 @@ To enable the enhanced history search, you'll need to install [fzf](https://gith
 #### macOS
 
 <details><summary>macOS instructions</summary>
+
 1. Download iTerm2 from [http://www.iterm2.com](http://www.iterm2.com) (optional). In my opinion, it is considerably nicer than the stock Terminal application that comes with macOS. There is an RCE flaw in all versions of iTerm 2 before 3.3.6, so update if you're using an affected version.
 2. Install the current version of Homebrew from [http://brew.sh/](http://brew.sh/).
 3. Install GNU Stow with `brew install stow`
@@ -125,11 +126,13 @@ To enable the enhanced history search, you'll need to install [fzf](https://gith
     1. In iTerm 2, go to Preferences->Profile in your iTerm 2 preferences, then select one of the Powerline-compatible fonts you just installed.
     2. **Make sure you also specify a Powerline-compatible font for non-ASCII in your iTerm 2 preferences or the prompt separators and branch glyphs will show up garbled**.
 7. Install `fzf` with `brew install fzf`
+
 </details>
 
 #### Linux
 
 <details><summary>Linux instructions</summary>
+
 1. Switch your shell to `zsh` with `chsh -s /bin/zsh`
 2. Install GNU Stow - `sudo yum install -y stow` on Red Hat / CentOS systems, `sudo apt-get -y install stow` on Debian / Ubuntu.
 3. Install `fzf` - `sudo apt-get install -y fzf` on Debian / Ubuntu, do a manual install on Red Hat / Centos - instructions are at [fzf](https://github.com/junegunn/fzf).
@@ -143,6 +146,7 @@ If the Powerline symbols can't be seen or are garbled, try closing all instances
 If you still can’t see the new fonts, confirm that the font has been installed to a valid X font path.
 
 If you get garbled branch glyphs, make sure there isn't a separate font setting for non-ASCII characters in your terminal application that you also need to set to use a Powerline-compatible font. Konsole needs to be set to use UTF-8 encoding, for example.
+
 </details>
 
 ### Set up Zgenom and the starter kit