Add IE8 Invalid Character Fix Configuration

This configuration is quite common for supporting IE8, though it's not a problem of select2 but caused by rails assets precompilation, I'm sure it will be asked again and again. It's time saving for new users if we can directly put them in README wiki.
Refer to https://github.com/argerim/select2-rails/issues/63, https://github.com/ivaynberg/select2/issues/1840