Remove notice about needing to sign on install

This notice is no longer necessary now that we've configured overcommit
to sign hooks when you install for the first time in 9cbffeb.

Author: sds

README.md

@@ -103,9 +103,6 @@ git init
 overcommit --install
 ```
 
-See the [Security](#security) section of the documentation to understand why
-`overcommit --sign` is necessary after installing hooks for the first time.
-
 After running `overcommit --install`, any existing hooks for your repository
 which Overcommit will replace will be backed up. You can restore everything to
 the way it was by running `overcommit --uninstall`.