I'm in the process of adding limit switches to my Shapeoko. Limit switches are nice because they give you two things:

1. They give you some added protection against driving a carriage off the end of your machine.
2. They give you the ability to set up an auto-homing routine. Auto-homing can be very helpful when you're doing a lot of repeat cutting against a fixed jig.

I found this 5-Pack of limit switches on Amazon for less than $5. That provides more than enough limit switches to set up an auto-zeroing system.

The mounts nestle inside the makerslide. The two holes should be tapped for M5 screws and bolted to the end-plates. This document describes how to connect limit switches to grbl.

Here's a video of the switches in action!

Edit
Small change: added a third hole to allow for mounting on the y-axis end-plate.

Tap the two holes for M5 screws. Then set inside the makerslide grooves and bolt to the Shapeoko end-plates.