README: Add blank lines around code blocks

markdownlink requires that there are blank rounds around code blocks.

Signed-off-by: Bjorn Andersson <[email protected]>

Author: quic-bjorande

README.md

@@ -18,12 +18,14 @@ make
 ### MacOS
 
 For Homebrew users,
+
 ```bash
 brew install libxml2 pkg-config libusb
 make
 ```
 
 For MacPorts users
+
 ```bash
 sudo port install libxml2 pkgconfig libusb
 make