readme: recommend setting indent-tabs-mode to nil

Author: tspiteri

README.md

@@ -7,6 +7,7 @@
 
 - [Installation via MELPA](#installation-via-melpa)
 - [Manual Installation](#manual-installation)
+- [Indentation](#indentation)
 - [rustfmt](#rustfmt)
 - [Tests](#tests)
 - [Other useful packages](#other-useful-packages)
@@ -44,6 +45,16 @@ Add this to your init.el:
 (autoload 'rust-mode "rust-mode" nil t)
 ```
 
+# Indentation
+
+The Rust style guide recommends spaces for indentation; to follow the
+recommendation add this to your init.el:
+
+```elisp
+(add-hook 'rust-mode-hook
+          (lambda () (setq indent-tabs-mode nil)))
+```
+
 # rustfmt
 
 The `rust-format-buffer` function will format your code with