Document _CreateError_ and _SendError_ on Readme

[AndersonQ]

I was integrating Raygun for the first time in a Go application, reading the documentation and the readme I could not find any reference to methods to send an error wich is not a panic. Which is quite misleading, as they exist. Beside Go applications hardly panic (CLIs usually do a lot more often than web servers/microservices). Thus I was considering raygun4go almost useless. I only found these methods out after have forked it and exposed them myself.

A section under Basic Usage could be added showing a example of them

The methiods I'm referring to are:

// Manually send a new error with the given message to Raygun. This will use the current execution stacktrace.
func (c *Client) CreateError(message string) error

// Manually send the given error to Raygun.
// If the given error is a "github.com/go-errors/errors".Error, then its stacktrace will be used in the Raygun report.
// For other errors, the current execution stacktrace is used in the Raygun report.
func (c *Client) SendError(error error) error

I'm happy to open the PR

[QuantumNightmare]

Thanks for the PR, I've merged that in now. I've also updated our site documentation to explain manually sending errors to Raygun.