pkg-config may need to be installed to install cargo-clone (or Xargo)

doc/Quickstart.md

@@ -70,6 +70,9 @@ $ rustup default nightly
 $ cargo install cargo-clone xargo
 ```
 
+> **NOTE** If the `cargo install` fails you may need to install `pkg-config`. In Arch this can be
+> accomplished with the `pacman -S pkg-config` command.
+
 ## Demo
 
 In the first part of the demo we'll use command line tools in the terminal then we'll transition to