Update README.md with rails error reporting example

[stillhart]

I think it would be helpful for beginners to give an example of the rails error reporting API. That hopefully gives them the hint to read and use the Rails API.

Maybe it's just me who likes to fly through the README code examples instead of reading it, but I already went into the source code to find out how to manually report one.

[fractaledmind]

We should link to the relaxin portion of the Rails Guide (https://guides.rubyonrails.org/error_reporting.html#using-the-error-reporter) and provide a bit more information here around the 3 different reporting methods and their behavior. Maybe we start by just using a block quote of that section of the Rails Guide

[fractaledmind]

Released with version 0.5.0