Merge pull request rbenv#1031 from mxl/brew-upgrade

Homebrew upgrade for HEAD package.

Author: mislav

README.md

@@ -58,9 +58,9 @@ Or, if you would like to install the latest development release:
 
     brew install --HEAD ruby-build
 
-Note, that if you chose to install the HEAD package, you need to reinstall it for every update:
+To upgrade the HEAD package use `--fetch-HEAD` option:
 
-    brew reinstall --HEAD ruby-build
+    brew upgrade --fetch-HEAD ruby-build
 
 ## Usage