Adding nvm exec command.

README.markdown

@@ -38,7 +38,7 @@ Often I also put in a line to use a specific version of node.
 ## Usage
 
 You can create an `.nvmrc` file containing version number in the project root directory (or any parent directory).
-`nvm use`, `nvm install`, and `nvm run` will all respect an `.nvmrc` file.
+`nvm use`, `nvm install`, `nvm exec`, and `nvm run` will all respect an `.nvmrc` file.
 
 To download, compile, and install the latest v0.10.x release of node, do this:
 
@@ -52,6 +52,10 @@ Or you can just run it:
 
     nvm run 0.10 --version
 
+Or, you can run any arbitrary command in the node environment:
+
+    nvm exec 0.10 node --version
+
 If you want to see what versions are installed:
 
     nvm ls