Installation docs unclear

[GuySartorelli]

silverstripe-hybridsessions/README.md

Lines 29 to 34 in 8ab99f7

	```php
	// Ensure that you define a sufficiently indeterminable value for SS_SESSION_KEY in your `.env`
	use SilverStripe\HybridSessions\HybridSession;
	HybridSession::init(SS_SESSION_KEY);
	```

If I was to actually copy this into my _config.php file, I'd get an error because no constant SS_SESSION_KEY has been defined.

What I assume is meant here is that the value for that environment variable be passed in, though it would be easy to read this as "pass in the name of the environment variable" instead, which would result in many different projects using the string "SS_SESSION_KEY" for encrypting their sessions.