Add Switch Versions of a Brew Formula as a zsh til.

README.md

@@ -129,6 +129,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Create A File Descriptor with Process Substitution](zsh/create-a-file-descriptor-with-process-substitution.md)
 - [Killing A Frozen SSH Session](zsh/killing-a-frozen-ssh-session.md)
 - [Search History](zsh/search-history.md)
+- [Switch Versions of a Brew Formula](zsh/switch-versions-of-a-brew-formula.md)
 - [Watch This Run Repeatedly](zsh/watch-this-run-repeatedly.md)
 - [Where Are The Binaries?](zsh/where-are-the-binaries.md)
 

zsh/switch-versions-of-a-brew-formula.md

@@ -0,0 +1,17 @@
+# Switch Versions of a Brew Formula
+
+If you've installed a couple versions of a program via brew and you'd like
+to switch from the currently linked version to the other installed version,
+you can use the `switch` command. For instance, if you are on version
+`1.8.2` of `phantomjs` and you'd like to switch to `1.9.0`, you can simply
+invoke:
+
+```
+$ brew switch phantomjs 1.9.0
+```
+
+More generically:
+
+```
+$ brew switch <formula> <version>
+```