Clarify quick start section in README

[mamachanko]

The quick start section of the README points to https://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/ saying:

We recommend you visit Spring Security Reference and read the "Getting Started" page.

However, there is no Getting Started page in the docs.

[rwinch]

Thanks for the report. I have made this part of our larger documentation efforts See gh-2567