Need to update menu-item instruction on readme as per v6

[karandatwani92]

Solves https://github.com/Laravel-Backpack/LogManager/issues/64

v6 menu command added

[welcome[bot]]

BOOM! Your first PR with us, thank you so much! Someone will take a look at it shortly.

Please keep in mind that:

• if this constitutes a breaking change, it might take quite a while for this to get merged; we try to emulate the Laravel release cycle as much as possible, so developers can upgrade both software once; this means a new big release every ~6 months;
• even if it's a non-breaking change, it might take a few days/weeks for the PR to get merged; unless it's a no-brainer, we like to have some community feedback on new features, before we merge them; this leads to higher-quality code, in the end; we learnt this the hard way :-)
• not all PRs get merged; sometimes we just have to hold out new features, to keep the packages lean; sometimes we don't include features that only apply to niche use cases;
• we're not perfect; if you think we're wrong, call us out on it; but in a kind way :-) we all make mistakes, best we learn from them and build better software together;

Thank you!

-- Justin Case The Backpack Robot

[welcome[bot]]

WHOOP-WHOOP! Congrats, your first PR on this repo has officialy been merged.

If you want to help out the community in other ways, you can:

• give your opinion on other Github Issues & PRs;
• chat with others in the Gitter Chatroom (usually for quick help: How do I do X);
• answer Backpack questions on Stackoverflow; you get points, people get help; you can subscribe to the backpack-for-laravel tag by adding a new filter; that will send you emails when new questions come up with our tag;

Again. Thank you for the PR. You are a wonderful person. Keep 'em coming :-) Cheers!

-- Justin Case The Backpack Robot

P.S. Help in the Backpack community is rewarded with free Backpack commercial licenses. It's the least we can do. If you feel you've helped the community with PRs, help & other stuff, please shoot Tabacitu an email and ask him if you qualify for free licenses. You scratch my back, I scratch your back. Thank you!