Provide INSTALL.txt

I wasn't sure how to install the module. It would be nice to provide a README.txt (or even the project page) put some instructions.

1. Download the file (the drush feature is awesome, btw)
2. Enable the module
3. Go to a view
4. Select pager options and choose infinite scroll

Step 4 was the one I couldn't figure out at first.

I'm not sure if any other instructions are necessary.