Update guide.xml

[marcobehler]

Hi all, a while ago I published a pretty comprehensive guide on logging in Java (https://www.marcobehler.com/guides/a-guide-to-logging-in-java) and was thinking - with the risk of it being rejected for self promotion - that including it in the 'Introduction' section would help more people get a clearer picture of the whole Java logging landscape and where and how JCL fits in.

In any case, thanks for considering!